Protection , Support & Movement in Animals

Introduction

An animal is wrapped in a protective covering, the integument, which may be as simple as the delicate plasma membrane of an Amoeba or as complex as the skin of a mammal. The arthropod exoskeleton is the most complex of invertebrate integuments, consisting of a two-layered cuticle secreted by a single-layered epidermis. It may be hardened by calcification or sterilization and must be molted at intervals to permit body growth. Vertebrate integument consists of two layers: the epidermis, which gives rise to various derivatives, such as hair, feathers, and claws; and the dermis, which supports and nourishes the epidermis. It also is the origin of bony derivatives, such as fish scales and deer antlers. Integument color is of two kinds: structural color, produced by refraction or scattering of light by particles in the integument, and pigmentary color, produced by pigments that are usually confined to special pigment cells (chromatophores). Skeletons are supportive systems that may be hydrostatic or rigid. The hydrostatic skeletons of several soft-walled invertebrate groupsdepend on body-wall muscles that contract against a non-compressible internal fluid of constant volume. In a similar manner, muscular hydrostats, such as the tongue of mammals and reptiles, and the trunk of elephants, rely on muscle bundles arranged in complex patterns to produce movement without either skeletal support or a liquid-filled cavity. Rigid skeletons have evolved with attached muscles that act with the supportive skeleton to produce movement. Arthropods have an external skeleton, which must be shed periodically to make way for an enlarged replacement. The vertebrates developed an internal skeleton, a framework formed of cartilage or bone, that can grow with the animal, while, in the case of bone, additionally serving as a reservoir of calcium and phosphate. Animal movement, whether in the form of cytoplasmic streaming, amoeboid movement, or the contraction of an organized muscle mass, depends on specialized contractile proteins. The most important of these is the actomyosin system, which is usually organized into elongate, thick and thin filaments that slide past one another during contraction. When a muscle is stimulated, an electrical depolarization is conducted into the muscle fibers through the sarcoplasmic reticulum, causing the release of calcium. Calcium binds to a protein troponin complex associated with the thin actin filament. This causes tropomyosin to shift out of its blocking position and allows the myosin heads to cross-bridge with the actin filament. Powered by ATP, the myosin heads swivel back and forth to pull the thick and thin filaments past each other. Phosphate bond energy for contraction is supplied by carbohydrate fuels through a storage intermediate, creatine phosphate. Vertebrate skeletal muscle consists of variable percentages of both slow fibers, used principally for sustained postural contractions, and fast fibers, used in locomotion. Tendons are important in locomotion because the kinetic energy stored in stretched tendons at one stage of a locomotory cycle is released at a subsequent stage.

Answer to Questions

Q.1.How would you define the following terms? What is the function of each?

i) Plasma membrane ii) Pellicle
iii) Epidermis iv) Cuticle
v) vii) Shell or test Skin vi) Tegument

In what group of animals would you observe each of the structure forming a protective covering?

Ans (i) Plasma Membrane Generally speaking, it is the outermost membrane of a cell. In protozoa it forms the external protective covering of the body. It consists of a phospholipid bilayer in which are embedded protein and cholesterol molecules. Its surface has molecules (regions) that detect changes in external condition’s, and act as a selective barrier to ions and molecules passing between the cell and its environment. In protozoa. The plasma membrane is a large surface area relative to body volume, so that gas exchange, up take of dissolved nutrients, and the removal of soluble wastes occur by simple diffusion.

(ii) Pellicle A pellicle or periplast is a thin, non-cellular covering of an animal, which may be composed of a cell membrane, cytoskeleton, and other organelle’s. In protists, such as, Paramecium it occurs just below the plasma membrane and is protective and supportive covering. Electron microscopy of pellicle, in Paramecium, shows that the pellicle consists of three membranes. The outermost one is continuous over the body surface and cilia; the innermost and middle membranes form a mosaic system of alveoli. The collective function of alveoli may be to confer a certain amount of rigidity on the otherwise flexible, and elastic pellicle, as a whole without preventing  at least localized flextures; however, they could also conceivably serve as a sort of buffer zone protecting the cytoplasm from substances which penetrate the outer cell membrane. pellicle help paramecium to maintain a definate and constant body shape. it also  transmits the force of cilia or flagella to the entire  body of the animal as it moves. fig .1.1

 

Protection , Support & Movement in Animals

Infracillature and pellicle of Paramecium.

iii) Epidermis An epidermis is a sheet of epithelial cells covering the external surface of a metazoan. It is derived from the embryonic ectoderm. In higher invertebrates, the epidermis (sometimes called hypodermis when covered externally by a non-cellular cuticle), rests on a basement membrane. Epidermal cells exposed at the surface of the animal may possess cilia as in larval forms of some flukes. The epidermis of some invertebrates also contains glandular cells, which secrete an overlying, non-cellular material that encases part or most of the animal (e.g. annelids, arthropods etc). Epidermis is largely a protective structure.

(iv) Cuticle: A cuticle is a non-cellular, protective, organic layer secreted by the external epithelium (hypodermis) of many invertebrates. The structure varies in different groups of animals — in rotifers and annelids, cuticles are thin and elastic; in aschelminths, the cuticle may bear spines, scales or other forms of ornamentation for protection; in most arthropods, cuticles are best developed, thick and rigid, and support the body. In the arthropod, the major bulk of the cuticle is a complex of a mucopolysaccharide, chitin, and proteins, and it may remain in this form, as for example, in the flexible but non-elastic cuticle of onychophorans, centipedes and insect larvae. In crustaceans and hexapods (insects), histological the cuticle is made of three distinct layers — an innermost endocuticle layer lying above the epidermis is the less hardened procuticle, a middle hardened procuticle, also called exocuticle (hardened by sclerotization or deposition of calcium carbonate). Exocuticle and endocuticle are collectively called procuticle. On the outer side lies the epicuticle, which is responsible for many of the unique features of the cuticle. In insects it consists of up to four discrete layers of proteins, polyphenols and phenolases. Above this lies a monolayer of closely packed wax molecules, oriented so that their water repellent groups point inward; this layer prevents the outward movement of water molecules and thus protects the animal from desiccation. Above this is a layer of randomly oriented wax molecules, topped by a hard, protective cement layer of lipoprotein. In crustaceans the procuticle is sometimes called the endocuticle. Fig. 1.2. 12

In crustaceans, the epidermis secretes uncalcified layers of pro cuticle. The layer outer to this uncalcified layer is the calcified layer. Outer to it and below epicuticle is the pigmented layer. A disadvantage of cuticle is that the animal has difficulty in growing within them. As a result, some of these invertebrates (e.g. arthropods) periodically shed the outgrown, old cuticle in a process called ecdysis or molting.

(v) Shell or Test: A shell or test is the outermost protective covering made of mainly inorganic material, such as, calcium. carbonate, silica, is often chitinous or pseudochitinous. In protozoa, tests are coverings in loose contact with the body e.g. in Lobosa, Filosa and Foraminifera. The enclosed protozoa can project out pseudopodia through one or many apertures in the shell or test. Corals, among the Cnidarians, have mucous glands in their epidermis, which secrete calcium carbonate shell. Shell in molluscs is the most developed protective case outside the body of gastropods, scaphopode bivalves, and Nautilus etc. The shell is secreted in three layers by the shell glands in margin  of the mantle epithelium. the outer layer of the shell is called the periostracum, which is a virously coloured horny layer formed of a chitin-like substance conchin or conchiolin. The middle layer of the shell, called the pristmatic layer is the thickest  and consists of calcium carbonate  alternating with organic matter. cells alomg the entire epithelial border of the mantle secrete the nacreous layer. fig 1.3 Among vertebrates, testudines possess a bony shell composed of dorsal portion called carapace, and ventral portion called plastron. It protects the animal from its predators. Fig. 1.13. 12

vi-Tegument: The outer covering of parasitic flukes and tapeworms   is  a  complex syncytium called a tegument. The outer zone of the tegument consists of an organic layer of proteins and carbohydrate called glycocalyx.The glycocalyx aids in transport of nutrients, wastes, and gases across the body wall, and protects the fluke against enzyme, and the hosts immune system. Also present in this zone are microvili that facilitate nutrient exchange. Nucleated cytoplasmic bodies and most of the organelles lie below the basement membrane.Slender cell processing called cytoplasmic  bodies with the outer zone of the tegument. fig 1.4

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(vii)       Skin: Skin is the vertebrate integument It is the largest organ with respect to surface area. Skin has two main layers — an outer epidermis; one to several cells thick epithelial tissue layer, and a dermls, which is thicker layer of  connective tissue beneath the epidermis. A hypodermis, consisting of loose connective tissue, adipose tissue, and nerve network, separates the skin from deeper tissues.

Q2.Give an account of general structure of vertebrate skin.

Ans.The skin of all vertebrates is composed of two principal layers — a thin superficial (outer) stratified epithelial layer, the epidermis, derived from ectoderm, and a deeper (inner) thick layer. the dermis, also called corium or true skin, which is of mesodermal origin.

The epidermis is stratified into two or more layers. The deepest layer, called stratum germinativum or Malpighian layer, rests on the dermis. The cells in this layer divide mitotically to renew cells that lie above. As the outer layers of cells are displaced upward by new generations of all cells beneath, the older cells are pushed outward which become flattened (squamous) and synthesize a tough water insoluble scleroprotein called keratin. Gradually keratin replaces all metabolically active cytoplasm. The cells die and are eventually shed; the process is called ecdysis or molting. This process is called keratinization, and the cells are said to be cornfield. The outer layer of these cornfield cells is hence called stratum corneum. The cells of stratum germinativum get their nourishment from the dermis. Some of the cells superficial to stratum germinativum become transformed into; mucous cells which produce mucous, poisonous secretions, and in some fishes, photophores; and into pertinacious cells, which may produce slime, enamel, and chiefly keratin — a main constituent of feathers, hair, nails, claws, reptilian scales and stratum corneum of terrestrial tetrapods.

The dermis is usually thicker than epidermis. and mainly serves a supportive role for the epidermis. It contains blood vessels, collagenous fibres, nerves, pigment cells, fat cells, and connective tissue cells called fibroblasts. These tissues support, cushion, and nourish the epidermis. The dermis commonly has an outer vascular spongy layer called stratum spongiosum, and a deeper, thicker stratum compactum. Fat is deposited between the body musculature and dermis in the layer called hypodermis. Fig. 1.5.

Skin_birdormammal-1

True bony structures, where they occur in the integument, such as scales in fishes, shell of turtles, and dermal bony plates are dermal in origin. Dermal bones also give rise to antlers as well as the bony core of horns in mamals. pigment cells or chromatophores, occur in any level of the skin in different vertebrate groups but tend to concentrate near the epidermis-dermis boundary.

Q.3.      Describe structure of skin in different groups of vertebrates.

Ans. The integument in vertebrates is built on the same fundamental plan however; the skin in various groups is modified according to their environment.

Cyclostomes/Jawless Fishes (Agnatha)

The epidermis is several cells thick. Dead stratum corneum is lacking. The outer cells are nucleated and secrete a thin cuticular covering. Several types of epidermal glandular cells may be present In hagfishes, multicellular epidermal slime glands produce a large amount of slimy mucous that covers the body surface. The slime protects the body from external parasites. Chromatophores present in epidermis impart color to the skin. Presence at Mucous  glands is an important adaptation for aquatic mode of life in fishes, and amphibians. FiG. 1 a

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Cartilaginous Fishes (Chondrichthyes)

The skin in cartilaginous fishes. is multilayered and contains many mucous glands and sensory cells. Small placoid scales are dermal in origin Placoid scales or denticles resemble vertebrate teeth, and contain blood vessels, and nerves. With the growth of body, the skin area also increases producing new denticles. Like vertebrate teeth once denticles reach maturity, they do not grow, instead wear down and are lost. Fig. 1.7,

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Bony Fishes (Osteichthyes)

The skin of most teleost bony fishes contains dermal bony scales These scales grow at the margins and over the lower surface forming lines of growth representing the number of seasons the fish has lived. The skin (dermis) is highly vascular and permeable and thus, especially in smaller fishes, functions in gas  exchange. The epidermis contains a number of mucous glands, the secretions of which called mucus help reduce friction and, prevent bacterial, and fungal infections. Some fishes have poison glands, or granular glands, these secrete irritating/poisonous alkaloids. Some fishes have photophores, and most have chromatophores. Both of these act like lures and warning signals. In between the dermis and muscles is the sub-cutis. Fig. 1.8.

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Color changes in different backgrounds are also observed. This ability to change color, known as metathesis is controlled by nervous and endocrine systems.

Amphibia

In amphibia the epidermis is composed of several layers of cells and is the first to have a dead stratum corneum, best developed in forms that spend a considerable time on land. The dead corneal layer is an adaptation to terrestrial life being protective, and prevents loss of excessive amounts of moisture. The dermis is thin, being composed of two layers; an outer loose spongy layer or stratum spongiosum, and an inner more compact layer or stratum 

compactum. Blood vessels, an aid to cutaneous respiration (even more important than lungs), lymph spaces, nerves and glands (mucous as well as poison glands in some forms) are abundant in stratum compactum.

Chromatophores (pigment cells) are abundant, present for the most part between epidermis and dermis, help in camouflage, or as warning coloration. Caecilians (apodans) have retained dermal scales. Fig. 1.9.

Reptiles The skin in reptiles is thick, dry, and almost devoid of glands (except scent glands for sexual activity). Absence of glands in skin is an adaptation to prevent evaporation of water. The stratum corneum of epidermis is well developed and produces horny scales that often form crests or spines, an adaptation to resist abrasions and protection. The dead outer layers are periodically cast off, a process called molting or ecdysis. The dermis is thick and has an upper layer with abundant chromatophores in lizards and snakes, and a lower layer with bundles of connective tissue. The color pigments in snakes and lizards form many color patterns for concealment or as warning colors

Fig. 1.10.

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Birds

The skin in birds is thin, loose, dry, and devoid of glands except uropygial or preen gland at the base of tail. The epidermis is delicate except on feet and shanks, where epidermal scales cover it. On other  parts  epidermis produces feathers. Feathers are derived from reptilian scales.

The dermis is thin forming toper and lower compact lever of collagen fibres and elastic fibres respectively. The skin is also devoid of chromatophores. Fat cells lie between upper collagen layer and lower elastic layer,  especially in aquatic birds. Fig. 1.11

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Mammals

The skin in mammals is highly developed. The notable features being the presence of hair, a greater variety and number of epidermal glands, a highly stratified, cornfield epidermis, and a derails many times thicker than the epidermis.

The epidermis has an outer layer of stratum corneum containing keratin. The (stratum corneum is specially thickened in friction areas forming callosities, in palms and soles and shoulders.

The epidermis is closely applied to the corneum (dermis) below just above the dermis is a layer of cells that divide continually and make up the stratum germinativum or Malpighian layer. As new cells form, they gradually migrate to the surface, become kerattnized and flat. The region where all this occurs is called the transitional layer. A granular layer, the stratum granulosum forms the lower part of transitional layer. Another transparent layer, the stratum lucidum lies just above the stratum granulosum in thick skins of palms of hands and soles of feet.

The dermis is thick and composed of connective tissue, nerves, smooth muscle fibres, blood vessels and certain glands (epidermal glands). In the lowest layer of epidermis are pigment cells. The epidermis in mammals forms hair, sudoriferous glands, sebaceous glands, mammary glands, nails, claws, horns, anthers, bill in duck billed platypus discussed in detail separately. Fig. 1.11.

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Q.4. Write a note on epidermal glands in various groups of vertebrates.

Ans. The epidermis in various groups of vertebrates gives rise to the epidermal glands of various types; luminous organs, and epidermal scales in reptiles, birds and some mammals, feathers in birds, and in mammals hair, nails, claws, horns, antlers, beaks and bills etc.

Epidermal Glands

Gland is a structure which produces a substance or substances essential and vital to the organism and species. The varied types of glands found in the skin are basically derived from the stratum germinativum alone; however they later migrate to the dermis. All of these are exocrine glands. The glands may be unicellular (mucous cells or goblet cells, granular gland cells and larger beaker cells of cyclostomes), or may be multicellular, classified as tubular glands (simple, coiled, branched or compound) and saccular glands (simple, branched, and compound). The gland is said to be; merocrine gland, when the cell body is not destroyed during secretion (sweat gland); apocrine when the acretions of gland gather at the outer ends of glandular cells, which is pinched off with a portion of the cytoplasm (just as budding, such as mammary gland); or holocrine or necrobiotic in which there is accumulation of secretion within the cell bodies The cells die and aro discharged with their contained secretions (sebaceous glands).

The various types of epidermal glands in various groups of vertebrates are as under:

Agnatha: Unicellular, merocrine mucous glands, that secrete mucin, a mucopolysaccharide, are the only integumentary glands found among the cyclostomes. The mucin combined with mucous is protective in nature; against microorganisms (pathogenic), fungus, dehydration by evaporation, and from predators.

Fishes: Both unicellular as well as multicellular mucous glands are found in the skin of cartilaginous and bony fishes. A few of the elasmobranchs and bony fishes have multicellular glands modified into poison glands specially associated with spines on the fins or on operculum e.g. in chimaeras, scorpion fish, zebra fish, Heteropneustus etc. Special glandular structures found in deep sea fishes are the luminescent organs (photophores that act as species recognition or act like lures and warning signals), and pterygopodial glands at the base of claspers in elasmobranchs.

Amphibia: Except for the unicellular glands of Leydig which occur in the epidermis of certain larval salamanders, the skin glands of the amphibians are multicellular and are either mucous glands (extremely numerous, an adaptation to keep the skin moist for cutaneous respiration), or poison glands. The parotid glands in the shoulder region of the common toad, for example, are actually masses of poison glands. Certain tropical frogs produce extremely potent poisons which were used by Indians for poisoning the tips of their arrows.

Reptiles: Correlated with the terrestrial mode of life and to prevent evaporation, and to prevent abrasions, there is marked development of the stratum corneum and formation of epidermal scales. There has been a significant reduction in the number of skin glands. However, some types of crocodiles and turtles possess musk glands, some snakes and crocodiles have glands associated with cloaca which produce a musky smelling secretion.

Birds: Like their reptilian ancestors, birds have a reduced number of skin glands. The most common ones is the uropygial or preen gland located at the base of tail, which secrete oil used in preening the feathers and making them water proof, specially in aquatic birds. A few birds also have modified oil glands associated with the external ear opening.

Mammals: The mammalian skin is extremely rich in glands e.g. there are about two and one half million sweat or sudoriferous glands on the human body. In man and several other mammals, such as, horse, the sweat glands by secreting sweat, a process called perspiration, regulate body temperature. In some mammals, certain sweat glands also produce pheromones. The mammary glands, which give this class its name mammalia, are thought to be modified sweat glands. Other type of glands in skin are the sebaceous (oil) glands, which are simple glands connected to hair follicles in the dermis. Their oily secretion called sebum lubricates, gives shine, and protects the skin. Sebum is a permeability barrier, an emollient, and a protective barrier against microorganisms. Ceruminous or wax glands in ear are also of sebaceous type. Some mammals, such as, carnivores, have scent glands around the anus, and sheep. pigs, and goats have these glands between the hoofs.

Male giant kangaroo and Hippopotamus secrete red coloured sweat.

Q.5. Write a note on scales in fishes (Dermal scales)

Ans. Dermal scales, as their name indicates are derived from the mesenchymal tissue of dermis. Dermal scales are present, for the most part in fishes and take the form of small bony or calcareous plates which fit closely together or overlap. Both epidermal and dermal scales are present in certain reptiles and in birds and some mammals in localized areas of the body.

Scales in Fishes

Not all the fishes have scales. The common catfish is an example. Scales in eels are very small and deeply embedded in the dermis. In certain fishes, e.g. chimaeras, scales occur in localized areas only. The scales in fishes are colourless Numerous pigments cells (chromatophores), which give the fish its colour, are located in the outer part of the dermis both above and below the scale.

Ancient vertebrates called Ostracoderms had a complete armour of bony plates Fig. The ancient exoskeletal scales, which persist even now in some primitive forms, are a four layered structure, i.e.

  1. Lamellar bone layer (lower layer).
  2. Spongy bony layer (which lies above lamellar bone).
  3. Dentine layer (a layer that lies above spongy layer).
  4. Enamel layer (the upper most layer).

The lamellar layer makes the bottom layer. This layer consists of a compact bone called isopedine. The next layer that is present above the lamellar layer is called spongy bone which contains numerous spaces for blood vessels. Above the spongy layer is the dentine layer, which in turn, is covered by a- layer of enamel.

The surfaces of the scales are mostly decorated with tubercles or ridges. Each tubercle has pulp cavity containing blood vessels. Between the enamel and pulp cavity the substance of the tubercle is formed of dentine.

Two types of scales were present in ancient bony fishes, i.e Cosmoid scales and Ganoid scales.

Cosmoid scales, contain cosmine instead of dentine. These are not found in any form living today. A modified form of cosmoid scale, however, is present in Latimeria and some primitive members of sub-class Sarcopterygii, i.e., Crossopterygii and Dippoi.

Ganoid scales contain ganoin and are found in primitive ray-finned fishes and these show two modified forms, i.e.

  1. Paleoniscoid Ganoid Scale exists today in Polypterus. This type of scale consists of isopedine layer, and uppermost layer of hard, shiny transluscent material called ganoin. Spongy layer is lost and cosmine layer is reduced.
  2. Lepisosteoid Ganoid Scales. In this type of scales both the spongy and cosmine layers are absent, the ganoin layer lies directly on isopedine layer These types of scales are present in Gar pike.

In modern fishes four types of scales are present, i.e.

(i) Ganoid, (ii) Placoid, (iii) Ctenoid and (iv) Cycloid scales (i) Ganoid Scales.

Ganoid scales contain ganoin as referred above. Ganoid scales, of Polypterus and Lepisosteus, fit together like tiles on a floor and are arranged in diagonal rows. Amia has modified ganoid scales on the head and sturgeons have similar scales in the tail region. Fig. 1.12 D, D.

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(ii)   Placoid Scales

Placoid scales are the diagnostic feature of elasmobranch fishes. Each scale consists of a basal bony plate (Isopedine) which is rounded in shape and embedded in dermis. From the plate a spine projects outward through epidermis and points posteriorily Fig. 1.12. The spine is made up of dentine covered with a hard layer of vitrodentine both of mesodermal origin. A pulp cavity lies within the spine and opens through basal plate through which blood vessels enter the pulp cavity. The scales of dog-fishes and sharks are numerous and set closely together, but in other forms they are usually large and scattered. The teeth on the rostrum or saw of sawfish are actually the large placoids embedded in sockets at their bases.

(iii)   Ctenoid Scales

Ctenoid scales are thin, translucent plates of bony material. Ctenoid scales have comb like spines at their free (distal) ends. Fig. 1.12 A.

Ctenoid scales are present in teleost fishes. Ctenoid scale can be compared with ganoid scales from which the ganoin has been lost. Each scale is embeded in a small pocket in the dermis. The scales are obliquely arranged so that the posterior end of one scale overlaps the anterior edge of the scale behind it. The basal end of ctenoid scale is usually scalloped. Lines of growth are present. These lines are used to tell the age of a fish.

(iv)    Cycloid Scale

A cycloid scale is semicircular in outline with concentirc lines of growth which may be used to determine the age of the fish. Cycloid scales are also located in pockets in the dermis but somewhat more loosely attached than,ctenoid scales. Cycloid scales are also present in teleosts. In some members such as flounders both ctenoid and cycloid types of scales are present. Fig. 1.12 B.

Write a note on scales in reptiles with special reference to testudines/chelonians.

Reptilia. Both dermal and epidermal scales occur in the reptiles but the underlying bony dermal scales are not as widespread as the overlying horny epidermal scales. Fig. 1.10. Some of the lizards have dermal scales (osteoderms), and the crocodilians have well developed dermal. bony plates along the back. Both the crocodilians and Sphenodon possess gastralia (pseudoribs), which are actually dermal bones developed on the belly-side of these animals. Turtles have highly developed, bony dermal scales which become intimately associated with the internal skeleton to contribute to the formation of the carapace on the turtle’s back and the plastron on its ventral side. Fig. 1.13.

Epidermal Scales are widespread in the reptiles and indeed are a chief diagnostic character of the class as a whole. The epidermal scales of lizards and snakes are folded one over the other like shingles on a roof, with the free edge of each scale projecting backward. On the other hand, the epidermal scales of the turtles and crocodilians develop separately, and each scale is continuous with those adjacent to it. The epidermal scales of the turtles not only overlie the carapace and plastron but also cover the soft parts such as the neck and legs. Epidermal scales are ultimately worn away or shed. The shedding process is known as ecdysis.

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Chelonian Armour

Chelonians (Turtles and Tortoises) have a specialized integumentary derivation called shell, which is formed of stratum corneal (epidermal) scutes and dermal bones. Dermal armour is an important protective adaptation of both extinct and

living reptiles. The chelonian armour however has surpassed all other reptilian classes in completeness and efficiency. All chelonian shells have horny (epidermal) scutes over the surface of the turtle body. Beneath these scutes there is a stout and effective bony shell. Fig. 1.13.

Chelonian shell consists of a convex oval dorsal shield called carapace and a flat belly slab called plastron.

Carapace consists of three sets of plates, i.e.

  1. Circling the margin of the carapace is a set of peripheral or marginal plates.
  2. From front to back in the midline is a series of subquadrate plates, most of which are fused to vertebral neural spines; hence are called neurals. The anterior and posterior members of neural plates are called pro-neural, nuchal and pygal respectively.
  3. Transversely placed between marginals and neurals are eight paired pleural (costal) plates bound to underlying ribs.

The elements of carapace are fused so completely, that some times even sutures become invisible, the lines of separation of the horny surface scutes, however, are often impressed on the surface of the bony carapace. In part the boundaries between the horny scutes of the carapace (and of the plastron as well) alternate with the sutures between bony plates, a condition which provides extra strength to the shell structure as a whole

Plastron of a typical chelonian consists of four paired plates and an anteromedian element. Their names are:

  1. A pair of epiplastron (clavicles) (ep).
  2. Median entoplastron (Inter-clavicle) (en).
  3. Hyoplastron (Hy).
  4. Hypoplastron (Hyp).
  5. XiphIplastron (X).

The three plates present at the anterior margin are actually elements (clavicles and interclavicles) of the dermal shoulder girdle, the remaining plastral elements, and all carapace plates appear to be new development in turtles. It is also suggestive that dermal abdominal ribs have been incorporated in the carapace

structure.                                                              •

Q.7. Write a detailed account on functions of integumentary system.

Ans. In the vertebrates, the skin through differentiation and specialization has become a multifunctional system. All known life activities of a vertebrate animal directly or indirectly are influenced by this system. Some of its important functions are described below.

1. Physical Protection: Since the integument completely encloses the body, it

is the system that largely protects the internal structures from the outside world. In addition to shielding the body from simple mechanical injury, the skin also protects it from invasion by foreign organisms and foreign substances.

Colour of the skin also plays an important role in protection. Colouration may be cryptic, or attractive. Cryptic colouration helps its animal to blend into its background and is concealed from potential predators. Attractive colouration helps in attracting the mating partner. If an animal is dangerous or noxious, though it may be brightly coloured, color acts as a warning to its potential predator (aposematic colouration). The colour of the birds and mammals is determined by

the colour of the feathers or of the hair. In fishes, amphibians and reptiles, colouration depends on the presence of pigment cells, chromatophores, in the integument. Some of these animals are able to change their colour to blend with changing backgrounds, partly by changes in the distribution of the pigment granules within the chromatophores and partly through changes in the position of the chromatophores within the skin.

  1. Protection from Injurious effects of sun light.

Most of the animals are protected from the injurious effects of ultra violet radiations in sun light by a cuticle, (in insects etc), scales (reptiles), feathers (birds) or fur and stratum corneum (mammals). About 90% of radiations are absorbed in the epidermis and only 10% reach the dermis.

Damaged cells in both the epidermis and dermis release histamine and other vasodilator substances that cause blood vessel enlargement in the dermis and the characteristic red coloration of sunburn. Light skins suntan through the formation of the pigment melanin in the deeper epidermis and by “pigment darkening,” that is, the photooxidafive blackening of bleached pigment already present in the epidermis. Unfortunately, tanning does not bestow perfect protection. Sunlight still ages the skin prematurely, and tanning itself causes the skin to become dry and leathery. Sunlight also is responsible for approximately 1 million new cases of skin cancer annually in the United States alone, making skin cancer the most common of malignancies among Caucasians. There is now strong evidence that genetic mutations caused by high doses of sunlight received during the pre-adult years are responsible for skin cancers that appear after middle age.

  1. Sensory Reception. Various sensory receptors (heat, cold, touch, etc.) by which the organism is informed of both inimical (harmful) and beneficial forces in the outer world are located in the integument which thus promote the adapative and survival value of the organism.
  2. Excretion and Chemical Homeostasis. Excretory functions are performed by several vertebrate integumentary structures. Among the fishes, some salt excretion takes place through the integument of the gills, and among the mammals elimination of salts is, in part, accomplished by the sweat glands of the skin.
  3. Respiration. Respiratory function of the skin is another important feature of a number o vertebrates, particularly the amphibians. They have a thin skin which is kept moist by the secretion of numerous mucous glands. Large blood vessels bring a good supply of blood close to the surface of the body. Exchange of gases (oxygen and carbon dioxide) takes place between the blood and air through the moist skin. One group of salamanders has become lungless and depends almost entirely upon the skin as respiratory organ (cutaneous respiration).

Thermo-regulation. Among the birds and mammales the skin and its derivatives (feathers and hair) serve a very important function in insulating the body against extremes of external temperature. The integument also acts partly as a thermostat, controlling the amount of heat that leaves the body by way of the skin. When the skin is cold, the small blood vessels in it contract (peripheral vaso-constriction), forcing the blood away from the surface of the body and thus reducing the amount of heat lost from the blood to the environment. When the skin is warm, the blood vessels present in the skin expand (peripheral vaso-dilation) so that more blood is brought close to the surface and more heat is dissipated

Evaporation of water secreted by the sweat glands is an important cooling mechanism in many mammals. Change of colour in the lower vertebrates also plays a role in temperature control. When skin colour is light, heat waves (far red) are reflected from the body of the animal; when the colour of the skin is dark, more heat waves are absorbed.

7      Osrno-regulation. Regulation of the water content of the body is, in part,

accomplished by the skin. In marine and terrestrial vertebrates which live in a hyperosmotic environment, the skin prevents the too rapid loss of body water to the surrounding medium, while in fresh water forms, which live in a hypo-osmotic environment, it prevents the absorption of too much water.

  1. Food Storage. Food storage is another significant function of the skin. Among the birds and mammals especially migratory birds and hibernating mammals, the fat present in the deeper layer of their skin acts as a reserve food supply.
  2. Nourishment. The mammary glands of the mammals are derivatives of the skin and hence are a part of the integumentary system. Newborn mammals depend upon the milk secreted by these glands for nourishment.

10 Locomotion. The use of the integument as a locomotive apparatus is widespread throughout the classes of vertebrates. The webbing on the feet of ducks and frogs, the fins of fishes, the wing feathers of the birds, the flight membranes of the bats, and the belly scales of snakes are among the locomotory structures derived from the integument.

11 Miscellaneous Functions. In addition to the functions mentioned above, a number of others can be assigned to the integument of certain groups. For example, in some of the mammals the skin is the site of the manufacture of vitamin D. Some of the mammals and amphibians have skin pouches called  marsupia which serve as brood pouches for the developing young. Among the birds, differences in colour and structure of the feather facilitate sex recognition.

Q.8.       Write a note on feathers.

Ans.     Feathers are highly modified epidermal scales. They are the outstanding feature of birds and are found nowhere else in the animal kingdom. There are three basic types of feathers: contour feathers (plumes); down feathers (plumules); and hair feathers (filoplumes). Contour feathers are the large ones that give the bird its general shape. Fig. 1.14.

300px-Feather_image

Figure 1.14 Anatomy of Selected Feather Types. 

Structure of contour feather

A typical contour feather consists of a long shaft and a broad flat portion called vane. The shaft consists of two parts, i.e., hollow quill or calamus which is embedded in the skin and a solid rachis which bears the vane, At the lower end of the quill is a small opening called inferior umbilicus. At the junction of rachis and quill is another opening called superior umbilicus. On the under side of the rachis and extending from the superior umblicus to the tip is a groove known as umbilical groove.

The vane is composed of a number of barbs which grow out (laterally) from the rachis. Each barb in turn bears (laterally) small barbules on both distal and proximal sides. The lower part of each distal barbule bears small barbicels which terminate in tiny hooklets or hamuli, which fasten on to the proximal barbules of next adjacent barb. Hooklets are absent from proximal barbules. As a result of this arrangement the barbs are hooked together through their barbules, and the vane offers a flat, wide, firm and unbroken surface except at the base where the distal barbules of a variable number of barbs lack hooklets. This part of the feather may then have a rather fluffy appearance. Should certain barbs become separated, the bird can easily put them in place again by preening or drawing the barbs together with its beak or bill.

In many birds at the junction of rachis and quill is located another feather called after-shaft or hyporachis. It bears barbs and barbules but lack hooklets. The after-shaft is generally Smaller than the main shaft and may consist only a small downy tuft. In some birds like Emu and Cassowary, the after shaft with its accessory parts may be as long as the main shaft and the feather appears to be double.

Down feathers are smaller than contour feathers, and the barbs branch out from the free end of the quill. They are found in nestling birds and under the contour feathers of adults.

Hair feathers or “pin feathers” have only the shaft with sometimes a small tuft of barbs at the end. They are usually scattered over the body surface, but may be concentrated around the mouth as “whiskers” as in fly cathchers, swallows.

Like the epidermal scales of reptiles, the feathers of the birds are replaced from time to time. Many birds particularly those citt the temperate, and arctic regions have a seasonal molt in which the feathers are shed and replaced within a few weeks as in pea-cock, or the replacement may be more gradual.

0. 9. Describe development of a contour feather? Feather Development

The development of all the different types of feathers follow the same pattern. Typically the development of down feather starts when an elongated but round dermal papilla develops in the skin. An annular groove, which is the beginning of the feather follicle, appear arround the papilla. During development the blood vessels of the dermal pulp supply nourishment to the growing structure Fig. 1.15.

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Figure 1.15

Formation of Bird Feathers during Embryonic Development. (a) Feathers form from epiderrnal evaginations (b) Blood flow to the feather supports initial feather development. Later in development, the blood supply to the feather is cut off .

The outer, thin cornfield epidermal layer forms a sheath called periderm. Below the periderm the epidermis gradually provides longitudinal folds or ridges which  extend into dermal folds. The ridges are produced by the activity of stratum germinativum at the base of papilla. The entire complex is called feather germ. The logitudinal epidermal ridges are going to give rise to barbs of the feather. It is at this time that pigmentation of the feather cell occur. The feather germ with its epidermal sheath grows rapidly and emerges out from the feather follicle above the surface of the skin as a pin-feather, the dermal pulp and stratum germinativum retract. At the same time the periderm at the tip splits and the tips of the cornfield epidermal ridges dry and crack. The distal ends of barbs become visible from the apex of periderm.. As the feather continues to grow, the proximal part of the elongated ridges become cornfield and separate. The periderm separates in the form of dandruff-like scales and the barbs spread out.

Q10-Write a note on hairs.                           

Hair is a unique mammalian structure which has no homology in other vertebrate. Hair function as tactile sensory structure and help to maintain most of the mammal’s endothermal homiothermy by insulating the entire skin when present as furcoat.

A. Structure and Development of Hair

Hair is entirely epidermal in origin. During development a small thickening in the epidermis, which is to become the hair follicle, from which the hair will arise, pushes down into the dermis and finally becomes cuped at its lower end. Connective tissue from the dermis extends into the cup like depression forming a  dermal papilla. The blood vessels , which extend into the papilla bring nourishment    to     the    stratum germinativum of epidermal cells. The follicle, which is at first a solid cord of cells, grows down in an oblique direction. The lower part of the follicle enlarges, this portion being called the bulb. Hair follicles usually are formed singly. Two thickenings, one proximal and one distal, appear along the side of a developing follicle. The proximal  thickening is destined to form a sebaceous gland; the distal thickening will provide the point of   attachment for one of the arrector pill muscles, which develop from surrounding mesenchyme.

As this growth process is going on, the epithelia, (epidermal ingrowth) undergoes a change so as to form a central shaft surrounded by a space between it and the follicular wall. This is actually the result of the gradual keratinization or cornificafion, of cells derived from the stratum germinativum. Keratinization of the shaft becomes complete about half the distance from the surface. As new cells continue to be formed and become keratinized, the central shaft increases in length and literally pushes its way through the solid cord of cells between shaft and the surface. Thus the hair emerges through a slightly depressed area which marks the place where the original invagination of epidermal cells occurred. In the meantime the proximal thickening, mentioned above, is proliferating and developing into a sebaceous gland. This opens directly into the hair follicle. Within the follicle, the hair shaft is surrounded by two layers of cells which do not extend beyond the limits of the follicle. The hair shaft is entirely cellular and is composed of a central core, or medulla; a middle cortex; and an outer covering, the cuticle, consisting of a single layer of scaly, cornfield cells which have lost their nuclei. The scales overlap, much like shingles on a roof, but the free edges point away from the skin, rather than toward it. At the base of the follicle the hair is expanded slightly to form a bulblike enlargement, which is sometimes called the “root” of the hair. All growth takes place in the root, where the cells Of the stratum germinativum divide actively. Beyond this point the cells gradually die and the shaft of the hair is thus composed of dead, cornfield cells. The character of the surface of the hair varies from smooth to rough and scaly.

  1. Growth

The manner in which hairs of various types grow is said to be either — definitive or angora. Angora hair grow to a considerable length before they loosen and are shed. The head hair of man are of this type. Definitive hairs, however, grow to a certain length and then stop. They are then shed and quickly replaced. Eyelashes, eyebrows, and body hairs are of the definitive type.

  1. Occurrence and Distributation of Hair

In most cases the entire skin is covered with hair but in some the hairy coat has disappeared from the most parts and only traces remain. During fetal development in all mammals the body is at one stage covered with a coating of fine hair called lanugo. This is absent only on ventral surfaces of hands and feet. The lanugo hair is transiet and is usually shed sometime before birth. A new growth of hair takes place over most of the rest of the body, forming a fine downy coat referred to as the vellus.

In cases where hairs have failed to develop, the condition is known as atrichosis. There are occasional human beings whose bodies are entirely devoid of hair. Abnormally excessive hairiness is called hypertichosis. This condition is frequently observed in men but rarely in women. In extreme cases the entire body of human being may be covered with a thick coating of hair like that of a dog or cat. The celebrated “Jo — Jo the dog-faced man” of circus fame is an example of this condition which is due to as pseudohypertrichosis.

  1. Colour

The colour of hair in various mammals actually shows very little range, and the vivid colours seen in other vertebrate classes are lacking. To be sure, in certain apes like the mandrills, highly coloured skin areas are present, but in these it is

the skin itself rather than the hair which bears the pigment. There are very few mammals that can be said to be highly coloured.

Four factors are responsible for the colour and luster of the hair; (1) the colour of pigment which is present in the cortex, (2) the amount of pigment, (3) the character of the hair surface, whether smooth or rough, and (4) the amount of air contained within the intercellular spaces of the medulla.

Pigment, the melanin, in hair is present in the intercellular spaces of the cortex. The amount that is present determines the shade. Browns seem to predominate in most forms, but in man the colour seems to have black and red pigments as its foundation.

  1. Shedding of Hair.

In mammals which show differences in their summer and winter coats, the hair is shed periodically. Horses and cattle shed their long, thick coats in the spring. The buffalo has a shaggy, moth-eaten appearance when large patches of hair are being shed. In many mammals, however, there is constant shedding and renewal.

  1. Denisty of the Hair Coat.

The density of hair on the body shows much variation in different mammals. Those living in cold climates, either far north or south or at high altitudes, possess the heaviest hair coats of all. Tropical forms are generally sparsely covered. A permanent abode in the water is associated with an almost total loss of hair in sirenians and cetaceans. Even among the races of man, rather marked variations exist. Certain members of the white race, the Aborigines of Australia and the hairy Ainus of the Yesso, are at one extreme. On the other hand, the natives of the Malay Peninsula with their smooth, prectically beardless skin furnish quite a Contrast.


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  1. Special Type of Hair.

The type of hair is different parts of the body of the same animal varies. Manes like those of the horse or lion, crests and tufts in certain regions, long tail hairs, eyelashes, eyebrows, dust-arresting hairs in the nose, all are hairs of special types which are, mainly adaptive in function. They serve as an aid in protection from enemies, in defense against insects, and in guarding delicate membranes from foreign particles which might otherwise be injurious. In man the head hair clearly serves the function of protection from sun and rain. Public and axillary hair aid in decreasing friction between the limbs, or between the limbs and body during locomotion.

In the category of special hair are the vibrissae (whiskers or feelers) found on the snouts of most mammals. These hair are unusually sensitive. They are best developed in nocturnal mammals and their follicles have an abundant nerve and blood supply at the base.

The porcupine is of special interest because practically all the main types of hair are to be found in this animal. The spines, or quills of the porcupine, which are really modified hairs, have barblike scale at their tips and are, loosely attached at the base. They cannot be projected but are easily pulled out when the barbed ends become embedded in the flesh of an enemy.

Differences in quality and distribution of hair in different regions are often quite marked in the sexes and furnish some of the outstanding secondary sex characters that distinguish male from female. The beards of men and the hairy

covering of the chest, arms, and legs are in marked contrast to the relatively smooth condition of the skin in women. The heavy mane of the lion clearly marks him from the lioness. Even in rats, where secondary sex characters are not very clear cut, the coarser texture of the fur of the male makes it easy to distinguish the sexes for anyone who has become accustomed to handling these animals in the laboratory.

0.11. Write a short note on beaks and bills.

Ans.     In turtles and tortoises and in all modern birds, teeth are lacking. Each jawbone is

covered with a modified epidermal scale which forms the beak or bill. In turtles and tortoises the beak is hard, and the bite of a large animal may be serious.

Among birds, great variation is to be seen in the shape of the beak correlated with its use in procuring food. Seed-eating birds usually have short, rather blunt beaks; insect eaters possess long and narrow beaks which are not so strong as those of the seed-eater. The long, strong, hooked beaks of birds of prey are well fitted for their methods of obtaining food.

As applied to birds, the words beak and bill are used interchangeably. The use of one or the other depends on the preference of the user. The term bill is more commonly used by ornithologists.

The bill of the egg-laying mammals, the duckbilled platypus, is soft and pliant. It is not covered with a modified epidermal scale and should not be confused with the type of bill found in birds.

Q.12. Write short notes on claws, nails and hoofs 

Ans. The hard structures which are present at the distal ends of the digits are derived from the horny layer of the integument. They differ from other epidermal structures in that they grow parallel to the surface of the skin, Wearing away occurs at the tip, Nails, claws, and hoofs are built upon the same plan, and even superficial observation reveals their homologies. The stratum ludidum of the epidermis is exceptionally well developed at thebase of these structures. True claws  Fig. 1.17. Above, wing of young hoatzin showing are present only in reptiles, birds, and     claws on first two digits; below, structure of a typical mammals. They have also been reptilian claw described in certain fossil amphibians. The South African “clawed toads,” Xenopus, Hymenochirus, and Pseudohymenochirus, are amphibians which have clawlike tips on the first three digits of the hind feet. Such structures may possibly foreshadow the appearance of claws in higher classes of vertebrates but they are not true claws. Mammals are sometimes grouped into those which bear claws, nails, or both, and those possessing hoofs. The hoofed animals are usually referred to as ungulates.

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Claws. A claw is composed of a dorsal scalelike plate called the unguis, and a ventral plate, the subunguis, or solehorn. The unguis is the better developed of the two and is of greater importance. The claw covers the terminal bony phalanX of the digit and is thus reinforced, Fig. 1.18. In the typical reptilian claw the unguis is curved both longitudinally and transversely and encloses the subunguis between its lower edges. The claw forms a sort of cap at the end of the digit. The outer layers of reptilian claws are shed and renewed periodically. Fig. 1.17.

Claws of birds are typically reptilian in structure, but many variations are to be found, associated with the mode of life of the bird. The strong, curved talons of the birds of prey; the sturdy, stubby claws of the gallinaceous birds; and the sharp, rather sledner structures of birds that perch or cling to surfaces are representative of the many types that may be observed. In birds, shedding of the entire claws is unusual, and growth and wearing away occur at a more or less constrant rate. Although claws are generally present only on the feet of birds, the young hoatzine, Opisthocomus cristatus of British Guiana Fig. 1.17 and the valley of the Amazon bears claws on the first two digits of the wings, and these are used as an aid in climbing. In this respect the young hoatzin resembles the fossil Archaeopteryx which had three clawed digits on each wing. As the bird grows older the claws disappear.

In the claws of mammals the subunguis is reduced in size and is continuous with the torus, or pad, at the end of the digit, which bear the friction ridges. Members of the cat family possess retractile claws which, when not in use or not extended, are withdrawn into a sheath. They are thus protected and kept sharp for the purposes for which the animals use them. In some mammals, as in certain lemurs, nails are present on some digits and claws on others. The tarsier has claws on the second and third digits of the hind feet, the others being supplied with nails.

Nails. In nails the dorsal unguis is broad and flattened, and the subunguis is reduced to a small remnant which lies under the tip of the nail (Fig. 1.18). The so-called “root” of the nail or region where growth of the unguis takes place, lies embedded in a pocket under the skin called the nail groove, or sulcus unguis. The nail bed lies beneath both the nail and its root. It is made up of proximal, middle, and distal regions, which show differences in structure. The proximal part, or matrix, is the most important. It is this portion which is concerned in the formation of the nail. Its anterior portion in man may be seen through the base of the transparent thumbnail and forms the whitish lunula, a crescent-shaded area. The lunula is not so conspicuous on the other digits. The stratum germinativum

above the base of the nail forms rather rough margin where the nail emerges from the sulcus unguis. This is called the eponychium, or cuticle.

Hoofs. In the hoofs of ungulates the unguis curves all the way around the end of the digit and encloses the subunguis within it (Fig. 1.18). The torus,. or pad, lies just behind the hoof and is called the frog. Since the unguis is of a harder consistency than the subunguis, it wears away more slowly and a rather sharp edge is thus maintained.

What are horns and antlers?

Except for the horn-like structures in few lizards, horns are found only in mammals. Certain dinosaurs possessed horns on the head, consisting of bony projections, each of which was probably covered with a horny epidermal cap. The so-called, “horned toads” of the genus Phrynosorna, indigenous to the southwestern United States, are lizards which have numbers of such structures on their heads. Each bony projection of the skull is covered with a horny epidermal scale, forming a rather sharp spine. They are of high protective value.

Among mammals, horns are found only in certain members of the order artiodactyla and in the rhinoceros of the order perissodactyla. Four types are recognized: the keratin-fiber horn, the hollow horn, the pronghorn, and the antler. Fig. 1.19.

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Keratin-fiber horns. The keratin-fiber horn is found only in the rhinoceros. It is a conical structure composed of a mass of hardened, keratinized cells growing from the epidermis covering as cluster of long, dermal papillae. A fiber, somewhat resembling a very thick hair, grows from each papilla. Cells which develop in the spaces between the papillae serve as a cement substance, binding the whole together. The fibers are not true hairs since their bases are not located in follicles extending into the dermis. These unusual and formidable weapons are median in their position on the head. In the Indian rhinoceros only one horn is present, but in the African species there are two, the larger one being in front. Fig. 1.19a.

Hollow horns. The hollow horn is the type found in cattle, sheep, goats, buffaloes, and others. In certain species they are found only in males, but this is not ture of all. The hollow horn consists of a projection of the frontal bone of the cranium covered by a cornfield layer of epidermis. A cavity, which is continuous  with the frontal sinus, extends into the bony projection. The horny layer is not shed. Fig. 1.19a.

Pronghorns. A unique type of horn, the pronghorn, is found only in  the   antelope, Antilocapra americana, of the Western United States. It consists of a projection of the frontal bone covered with a horny epidermal sheath. The sheath usually bears one prong although as many as three  have        been observed. The unusual

feature of this type of horn is that the horny covering is shed with annual periodicity and a new horn forms from the epidermis which persists over the bony projection. Fig. 1.19.

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Antlers. The male members of the deer tribe possess branched antlers which project from the frontal bones. Only in the reindeer and caribou do both sexes bear antlers. In its fully developed state an antler is composed of solid bone. It is therefore entirely mesodermal in origin and properly should not be called a horn at all.. Antlers are to be considered as parts of the dermal skeleton. Since, however, the antler is formed under the influence of the integument, it is expendient to include it under the discussion of integumentary derivatives.

In giraffe both sexes possess antlers. These, however, are small and rather inconspicuous. These are never shed and remain permanently in velvet. Antlers are secondary sex characters in other species with the exception of the caribou and reindeer. They reach the height of their development just before the mating season, when it is customary for the males to fight for domination of the herd. Occasionally the antlers of two bucks become interlocked in combat in such a manner that they cannot be extreicated. Starvation and death of both combatants usually follows.

Antler growth is apparently regulated by hormones secreted both by the testes and the anterior lobe of the pituitary gland.

0.14. Write a short note on Mammary Glands,

Ans. The mammary glands, give the name to class mammalia for vertebrate animals possbssing mammary or milk producing glands. The mammay glands produce milk for the nourishment of the young — a unique feature found only in mammals.

Mammary glands are highly specialized and modified derivatives of “Sweat Glands” with apocrine secretory nature. Mammary glands develop in both sexes from a pair of elevated bands of ectoderm known as milklines, which extend along the ventro-lateral body wall of fetus extending from the base of the pectoral limb up to the pelvic limb bud. Depending upon the species, mammary buds appear at various pointvalong the mammary ridge, each bud develops into a separate mammary gland with nipple or teat as the case may be. Nipples or teats are usually paired, their number being roughly proportional to the number of  young delivered at birth. They vary from one pair in man and some others to six pairs in certain insectivores. If more than usual numbers of mammary glands “Breasts” occur, the condition is called hypermastia or polymastia. An interesting record is held by a Polish woman who possessed eight functional breasts. If only supernumerary (more than normal number) nipples are present, the term hyperthelia is used. Hyperthelia is not uncommon in man.

Actively secreting breast is made up of many small masses called lobules. 

Each Adipose tissue lobule, in turn consists of large number of milk secreting units called lobule,Each lobule, in turn consists of large number of milk secreting units called alveoli.The small ducts leading from the alveoli in a lobule gradually converge to a large  duct. This generally unites with similar ducts from other lobules, and the common ducts thus formed lead to outside. In  the inactive mammary glands the alveoli and lobules are reduced or  absent’ and the glandular tissue consists mainly of branching ducts. Since adipose (fatty) tissue usually surrounds the ducts and alveoli, this contribute much to the size of mammae or breasts. Fig. 1.20.

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In Prototherians mammary glands have no nipples, the milk ducts instead, open directly on to the surface of skin. The young animals get milk by lapping, licking or sucking. In metatheria and eutheria the mammary ducts lead to nipples or teats which are grasped by the mouth of young when sucking.

A nipple is a raised area on the breast through which mammary ducts open directly to out side. A false teat on the other hand, is formed when skin of the mammary area grows outward to form a large projection in animals like horses, cattle and other. The mammary ducts open into a large space called “cistern” at the base of a teat and the milk is then carried by the secondary duct or tube to the surface. Some mammals like cattle, goats and horses etc. possess udder. The udder is made up of mammary and other tissues through which the mammary ducts ramify and converge at the base of the teats.

Although nipples, teats and mammae may be present in both sexes, functional mammary glands occur normally only in lactating females. In exceptional cases, functional mammary glands have been known to occur in males. Such a condition is called gynecomastia.

15. What determines color of skin?! Write a note on Integument coloration.

Ans. The colors of animals may be; vivid and dynamatic when serving as important recognition marks, or as warning coloration, or may be subdued or cryptic when used for camouflage.

Skin color is usually produced by pigments, but in many insects and in some vertebrates, especially birds, certain colors are produced by the physical structure of the surface tissue, which reflects certain wave lengths of light and eliminate others. Colors produced in this way are called structural colors, and are responsible for the most beautifully iridescent and metallic hues found in animal kingdom. Certain structural colors of feathers are caused by minute, air-filled spaces or pores that reflect white light or some portion of the spectrum. Iridescent colors that change hues as the animal’s angle shifts with respect to the observer are produced when light is reflected from several layers of thin, transparent film.

Pigments or biochromes contained in large cells called chromatophores, more common in epidermis/dermis boundary, impart color to integument. The most widespread of animal pigments are the melanins, which is a group of black or brown polymers. Yellow and red colors are caused by carotenoid pigments frequently contained within special pigment cells called Xanthophores. Two entirely different classes of piOments called ommochromes and pteridines are usually responsible for the yellow pigments of molluscs and arthropods. Green colors, rare among animals are due to yellow pigments overlying blue structural color. Iridiophores, contain crystals of guanine or some other purine rather thar pigment impart a silvery or metallic color by reflecting light.

Skin coloration is either cryptic or attractive.

Q. 16. What do you mean by skeleton in animals? What are, its types and functions in animals? What animals possess these skeletons?

Ans.     Skeletons are supportive systems that provide rigidity to the body, provide surfaces for muscle attachments, and protect some important soft-organs of the body. There are three main types of skeletons: hydrostatic skeletons, exoskeletons, and endoskeletons-the later two also called as rigid skeletons.

A hydrostatic skeleton consists of fluid held under pressure in a closed body compartment. A hydrostatic skeleton is well suited for life in aquatic environment. as in cnidarians, flatworms, nematodes and annelids.

An exoskeleton is a hard encasement deposited on the surface of an animal. For example, most mollusks are encased in calcareous shells secreted by the mantle, arthropods have an exoskeleton (cuticle) composed of chitin (a polysaccharide similar to cellulose).

An endoskeleton consists of hard supporting elements such as, bones, buried within the soft tissues of an animal e.g., sponges have calcareous or siliceous spicules echinoderms have ossicles composed of magnesium carbonate and calcium carbonate crystals, chordates have cartilage, bone, or some combination of these tissues.

Functions Some of the important functions of skeleton in animals are

(i) The skeleton gives support and rigidity to an otherwise soft body or body part of an animal. Most land animals will sag from their own weight if they had no skeleton to support them.

(ii)       Skeletons give a characteristic form or shape to the body or body organ of an animal.

(iii)      Hard skeletons protect soft tissues/organs of the body e.g.; vertebrate skull protects the soft brain, ribs form a cage around the head, lungs and other internal organs.

(iv)      Skeletons provide the series of firm, hinged segments, that are essential in conjunction with the muscles, for movement or locomotion.

(v)       Endoskeletons serve as a store for various minerals such as, calcium and phosphate.

(vi)      Bones, specially flat and long bones play important role in manufacture of red blood cells and some white blood cells.

(vii)   Exoskeletons protect the body from enemy attacks e.g shell in mussels, snails, tortoise etc.

Q. 17. What is a hydrostatic skeleton? Which animals have hydrostatic skeleton?  How does it function?

Ans. A hydrostatic skeleton consists of fluid (water or body fluid such as, blood or coelomic fluid) surrounded by a tension-resistant sheath of longitudinal and/or circular muscles. The hydrostatic skeleton prevents the body from collapsing when its muscles contract. The hydrostatic skeleton may cushion internal organs from shocks and provide support for burrowing and crawling.

Hydrostatic skeleton is the main type of skeleton in some cnidarians (Hydra) flat worms   (Planarians),

nematodes and annelids. In cnidarians and planarians it  is the gastrovascular cavity that functions as hydro static skeleton, in nemedians

it is the rhynococle, in as chelminthes it is the pseudococelom, in annelids it is the coelom and in mollusks it is the hemocoel. Fig. 1.21.

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Mechanism Cnidarians, such as Hydra or a jelly fish, control their form and movement by using contractile cells (which act as muscles) to change the shape of gastrovascular cavity. Because water cannot be compressed very much, decreasing the diameter of the cavity forces the body to increase in length.

In flatworms, such as in planarians, the interstitial fluid is kept under pressure and functions as the main hydrostatic skeleton.

Roundworms (nematodes) hold the fluid in the body cavity, which is pseudocoelom, under high pressure, and contraction of longitudinal muscles result in thrashing movements.

In earthworms and other annelids, the coelomic fluid functions as a hydro static skeleton. The coelomic cavity is divided by septa between the segments of the worm’s body, and thus animal can change the shape of each segment individually using both the circular and longitudinal muscles. The hydro static skeleton enables earthworms and most other annelids to move by peristalsis.

Q. 18. What are exoskeleton and endoskeleton? Give examples. Give advantages and disadvantages of both types of skeleton.

Ans.An exoskeleton is usually referred to a
rigid (as in mollusks) or jointed and move able (as in arthropods), hard encasement deposited on the surface of an animal. The exoskeleton in invertebrates is mainly protective, but it may also perform a vital role in locomotion. An exoskeleton may take the form of an outer calcareous shell as in mollusks. As the animal grows, the mantle secretes or adds the shell to the previously formed shell at its edges. The exoskeleton in arthropod is a cuticle which is a non-living coat secreted by the epidermis. About 30 – 50% of cuticle consists of chitin-a polysaccharide similar to cellulose. Cuticle is hardened due to organic compounds. The exoskeleton of an arthropods must periodically be shed (molted) and replaced by a larger case with each spurt of growth by the animal. Fig 1 22.

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The arthropod type of exoskeleton is perhaps a better arrangement for protection and support for small animals than a vertebrate endoskeleton because a hollow cylindrical tube can support much more weight without collapsing than can a solid cylindrical rod of the same material and weight. Arthropods can thus enjoy both protection and structural support from their exoskeleton. But for larger animals hollow cylinder would be completely impractical. If made thick enough to support the body weight, it would be too heavy to lift; but if kept thin and light, it would be extremely sensitive to buckling or shattering on impact.

The endoskeleton consists of hard supporting elements, such as bones, buried within the soft tissues of the animal. Although whenever one speaks of an endoskeleton, we take for granted that vertebrate endoskeleton is being discussed. However some forms of endoskeleton are also present in some invertebrates e.g., sponges have endoskeleton composed of siliceous or calcareous spicules. Similarly echinoderms have hard plates called ossicles composed of magnesium carbonate and calcium carbonate crystals, and the separate plates are bound together by protein fibres. The vertebrate endoskeleton is composed of bone and cartilage. Protochordate larvae and embryos of all vertebrates possess a semi rigid supportive axial rod (notochord) composed of large vacuolated cells surrounded by layers of elastic and fibrous sheaths.

The endoskeleton grows with the growth of the body of a vertebrate. Bony endoskeleton is much heavier as compared to cartilaginous endoskeleton and chitinous exoskeleton ot arthropods.

O. 19. How do cartilage and bone differ structurally and chemically?

Ans.     Cartilage is a firm but flexible specialized type of connective tissue that provides a site for muscle attachment, aids in movement at joints, and provides support.

Bone (osseous) tissue is a specialized connective tissue in which calcium phosphate, calcium carbonate and other organic salts are deposited in the matrix. It is more rigid that provide a point of attachment for muscles and transmit the force of muscular contraction from one part of body to another during movement. In addition, bones of the skeleton support the internal organs of many animals, store reserve calcium and phosphate, and manufacture red blood cells and some white blood cells. Fig. 1.23.

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Differences Cartilage and bone are specialized connective tissues in which inorganic salts and protein fibres have been deposited in the matrix. They differ in cell type (chondrocytes in cartilage, osteocytes in bone), in composition of the matrix (chondroitin sulfate in cartilage, calcium phosphate in bone), and in vascularization (cartilage is a vascular bone is vascular). They also differ in their microarchitecture: bone can be highly ordered into osteons and cartilage is less organized.

On their surfaces, both the cartilage and bone are covered by a similar coat of fibrous connective tissue. Although virtually identical, these fibrous sheaths are logically termed the perichondrlum around cartilage and the periosleum around bone.

Q. 20. What are different types of cartilage and bone?

Ans. Cartilage Cartilage is a firm but flexible connective tissue. The matrix primarily consists of chondroitin sulphate (ground substance) and collagenous or elastic proteins (fibres). Spaces within the matrix called lacunae house cartilage  cells or chondrocytes. 

The physical properties and its functional roles are determined largely by the type and abundance of protein fibres in the matrix. There are three types of cartilaginous tissue. i.e., hyaline cartilage, fibrocartilage and elastic cartilage. Fig. 1.24.

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i) Hyaline Cartage. The name hyaline meanings “glassy” refers to the homogenous appearance of the matrix.It is transluscent and of bluish green cast. Collagen fibrils are present in the matrix but not in abundance. In embryos it makes up many bones before they undergo ossification. In adults it persists at the tips of ribs, In tracheal rings, many parts of skull.

(ii)      Fibro/Fibrous Cartilage Fibro or fibrous cartilage has little intercellular substance, the solid ground substance is reinforced with collagen fibres that is  effective in resisting compressive forces. It occurs in intervedebral disks, the pubic symphysis, disks within the knee etc.

(ii)      Elastic Cartilage It is flexible and springy, a property due to the presence of elastic fibres in the matrix. It is present in the ear pinna of mammals, epiglottis etc.

Bone bone forms the greater part of vertebrate skeleton. Matrix is impregnated with certain inorganic salts such as, calcium carbonate, calcium phosphate, sodium chloride and magnesium phosphate, which give the bone rigidity and hardness. In many vertebrates, these salts are arranged in a regular and highly ordered unit called osteon, forming osteonic canals or Haversian system. Each osteon is a series of concentric rings made of bone cells and layers of bone matrix around a central canal (osteonic canal) through which blood vessels, hymphatic vessels, and nerves travel. Bone cells called osteocytes lie in spaces called lacunae, scattered throughout the matrix. Osteocytes have numerous irregularly branching processes which penetrate through boundaries of lacunae into matrix through small canals called canaliculae. The adjoining osteonic canals are interconnected diagonally by communicating canals or Volkmann’s canals. The layers of bone matrix are termed lamellae. Fig. 1.23.

There are various criteria for classification of bone. From its visual appearance, we see that two types of bone: cancellous or spongy bone, which is porous, and compact or periosteal bone, which is dense.

(i)   Spongy or Cancellous bone ensures considerable strength with a

minimum weight. It consists of small bony plates and bars joined together, with small spaces filled with bone marrow.

(ii)Compact bone appears as hard solid mass without any spaces except microscopic ones. It forms hard tubular shaft of the typical long bones and encloses the marrow cavity. Compact bone is externally lined with dense layer of connective tissue, the periosteum, with which muscles and tendons are attached. The cavities of bone filled with bone marrow are lined with a thin layer of connective tissue, the endosteum.

Q 21. How do the endoskeletons of a fish and a frog differ?

Ans.  Since water has a buoyant effect on the fish body, the requirement for skeletal support is not as demanding in these vertebrates as it is in terrestrial vertebrates such as, in frogs or toads, reptiles, birds and mammals. Jawless fishes have a vague or no vertebral column. Most jawed fishes  however have an axial skeleton that include a notochord, ribs ,and cartilaginous or bony vertebrae. Muscles used in locomotion attach to the axial skeleton. In fishes the fins are the organs for locomotion. Fig. 1.26.

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Tetrapods (frogs etc.) must lift themselves to walk on land. They need support to replace the buoyancy of water as enjoyed by fishes. In tetrapods such as frog, the endoskeleton became modified for support and locomotion on land. The following modifications evolved:

(i) The intervertebral discs evolved that articulate with adjoining vertebrae. The intervedebral discs help hold the vertebral column together and also absorb shock and provide joint mobility.

Bone replaced cartilage in the ribs, which became more rigid.

(iii)       The various types of connective tissue that connect to the axial skeleton help keep elevated portions of body from sagging.

(iv)        Appendages became elongated for support on a hard surface, and changes in shoulder girdle enabled the neck to move more freely.

Q. 22. What are the two major divisions of the human skeleton? Name the major components of each division.

Ans. The human endoskeleton  has two major parts: the appendicular skeleton. Fig.1.27.

detailed-human-skeleton-diagrams

Figure 1.27 Human skeleton. Dorsal view. In comparison with other mammals, the human skeleton is a patchwork of primitive and specialized parts. Erect posture, brought by specialized change in legs and pelvis, enabled the primitive arrangement of arms and hands (arboreal adaptation of human ancestors) to be used for manipulation of tools. Development of the skull and brain followed as a consequence of the premium natural selection put on dexterity and to appraise the environment.

Axial skeleton: The axial skeleton is made up of the skull, vertebral column, sternum and ribs.

Skull: The skull is composed of eight bones that form the cranium, and fourteen facial bones. The cranium protects the brain. It contains two paired bones-the temporal and parietal, and four unpaired bones – frontal, occipital, sphenoid and ethmoid. The facial bones are-six Raked bones;    the maxillae, zygomatic,  nasal,lachrymal, palatine, and inferior concha, while two unpaired bones are mandible and vomer.

Vertebral column: It is composed of thirty three vertebrae – seven cervical,twelve  thoracic,  five lumbar, five sacral and four coccyg ial.

Ribs: There are twelve (one in 20 has 13, Hickman) pairs of ribs, ten of which hare connected to sternum while two are floating ribs.

Appendicular skeleton: The appendicular skeleton is composed of pectoral girdle, fore or upper limbs or arms, pelvic girdle, and hind/lower limbs or legs.

The pectoral girdle is composed of scapula and clavicle.

The upper limb is composed of a humerus, a radius bone, ulna, eight carpals or wrist bones, five metacarpals and five phalanges.

The pelvic girdle is composed of fused ilium, pubis and ischuim forming os coxa.

The lower limb is composed of a femur, tibia, fibula, eight ankle bones Or tarsals, five metacarpals and five rows of phalanges.

Q. 23. Considering protists and animals, where would you find amoeboid movements?

Ans. Amoeboid movements are non muscular movements observed among protists such as in Amoeba. Amoeboid movements also occur in certain white blood cells, coelomic cells, in embryonic tissue movements, in wound healing„ and in many cell types growing in tissue culture.

Q24-What is the major difference in cilia and flagella?

Ans-Cilia     and    flagella  are
elongated appendages on the surface of some cells by which the cells propel themselves or move material over the cells surface. Structurally          cilia         (L.”eyelashes”) and flagella (L. “small whips”) are similar (a 9 + 2 pattern of microtubles). The cilia however, are shorter and numerous, whereas flagella are 5 — 20—times as long as cilia, and occur singly or in pairs. The main difference between a cilium ada flagellum is in their beating pattern. A flagellum beats symmetrically with snake like undulations so that water is propelled parallel to the long axis of the flagellum. A cilium in contrast, beats asymmetrically with a fast  followed by a slow recovery power stroke in one direction during which the cilium bends as it returns to it’s a original  position. Thus water is propelled parallel to the cilia ted surface. Fig. 1.28.

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Internal Structure of Cilia and Flagella. In cross section. the boring doublet and spokes extend toward the central paired microtubutes. The dynein arms push against the adjacent microtubule doublet to bring about movement

Q25-How would you describe amoeboid movement?

Ans- Amoeboid movement was first observed in Amoeba. The plasma membrane of an Amoeba has membrane-adhesion proteins that attach temporarily to the substrate to provide traction, enabling the cell to crawl steadily forward. New pseudopodia form and attach to the substrate. The plasma membrane also seem to slide over the underlying layer of cytoplasm when an Amoeba moves The plasma membrane rolls in a way similar to a bulldozer track rolling over its wheels. A thin fluid layer between the plasma membrane and ectoplasm (hyaline cap) facilitates this rolling.As an Amoeba moves, the fluid endoplasm flows forward into the fountain zone of an advancing pseudopodium. As it reaches the tip of apseudopodium endoplasm changes into ectoplasm. At the same time, ectoplasm near the opposite end in the recruitment zone changes into endoplasm and begins flowing forward. Movements depend on actin, myosin and probably some other regulatory proteins. Fig. 1.29.

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Q. 26. How would you describe ciliary creeping?

Ans.     Cialiary creeping is the principal mean of locomotion                in flatworms and
nemertimes. The epidermis of free living flatworms (e.g. turbellarians) and nemertines is abundantly ciliated. Turbellarians are primarily bottom dwellers that glide over the substrate. As they move, they lay down a sheet of mucus that aids in adhesion and helps the cilia gain traction. The densely ciliated ventral surface and a flattened body enhances the effectiveness of this locomotion.

O. 27.    What are some physiological properties of muscle tissue?

Ans.     Muscular tissue is the driving force, the power behind movement- in most
invertebrates and the vertebrates. The basic physiological property of muscle tissue is contractility. In addition, muscle tissue has three other important properties: (i) excitability or irritability, the capacity to receive and respond to stimulus; (H) extensibility, the ability to be stretched; and (iii) elasticity, the ability to return to its original shape after being stretched or contracted.

O. 28.    What are the three types of muscles?

Ans.     On the basis of general microscopic appearance, the muscles are classified into
three types: (i) skeletal (ii) cardiac, and (Hi) smooth muscles. All muscles come with standard cellular equipment, namely nuclei, mitochondria, but specialized terms have grown up for familiar cell organelle e.g., sarcolemma for smooth endoplasmic reticulum etc. Fig. 1.30.

(i)        Skeletal Muscle Skeletal muscle is a striated voluntary muscle the contractions of which are controlled by nervous control. Skeletal muscle fibres are multinucleated and striated (transversely striped, with alternate dark and light bands). Skeletal muscles attach to skeleton.

(ii)    Cardiac Muscle Cardiac muscle fibres are involuntary, have a single

nucleus, are striated (have light and dark bands), and are branched and join to each other by distinct intercalated discs into sheets. This branching allows the fibres to interlock for greater strength during contraction. Cardiac muscles relax completely between contractions, that is why hearts do not fatigue.

(iii)   Smooth Muscles Smooth muscle lacks the striations typical of skeletal

muscle. The cells are long, spindle shaped tapering strands, each containing a single nucleus. They are arranged in a parallel pattern to form sheets. The cells contract slowly, but can sustain prolonged contractions and does not fatigue easily. These are present in alimentary canal, blood vessels, respiratory passages, and urinary and genital ducts. Its contractions are involuntary.

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Q. 29. What is an asynchronous invertebrate muscle?

Ans Asynchronous muscles are found in more specialized insects (bees, wasps, flies,
beetles). Their mechanism of action is complex and depends on storage of potential energy in resilient parts of the thoracic cuticle. As one set of muscle contracts (moving the wing in one direction), they stretch the antagonistic set of muscles, causing them to contract (and move the wing in other direction). Because muscle contractions are not phase-related to nervous stimulation, only occasional nerve impulses are necessary to keep the muscles responsive to alternating stretch activation. Thus extremely rapid wing beats are possible. For example, butterflies (with synchronous/direct) muscles may beat as few as four times per second, insects with asynchronous muscles may vibrate at 100 beats per second, in midge (a dipteran related to flies and mosquitoes this can happen a thousand time per second. For details of mechanism of synchronous and asynchronous flight mechanism, consult book for paper B, chapter 9, question 6 and 7.

Q. 30. What invertebrates move by (i) pedal locomotion, (ii) looping movements, (iii) water-vascular system (iv)walking (v)jumping (vi)flying? Give brief description of each.

Ans. (i) Pedal Locomotion

Flatworms, some cnidarians, and gastropod mollusks move by means of waves of activity in the muscular system that are applied to the substrate. This type of locomotion is termed as pedal locomotion.

(ii)     Looping Movements

Looping movement is exhibited by leeches and some catterpillars, (insect larvae). Leeches have anterior and posterior suckers that provide alternate temporary points of attachment. Lepidopteran catterpillars exhibit similar locomotion in which arching movements are equivalent to the contraction of longitudinal muscles. Fig. 1.31.

531744_orig

Movement relative to the ground

Figure 1.31 Looping Movements. (a) Leeches have anterior and posterior suckers, which they alternately attach to the substrate in looping movements to move forward (b) Some insect larvae such as lepidopteran caterpillars, exhibit similar movements. The caterpillar uses arching movements to move forward.

(iii)     Water-Vascular System .A water-vascular system is a characteristic feature of echinoderms. For example, sea starts typically have five arms, with a water-vascular canal in each. Along each canal are reservoir ampullae and tube feet. Contraction of the muscles in ampullae drives water into the ampullae. Thus, the tube feet extend by hydraulic pressure and can perform simple step-like motions.

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Figure 1.32 Water-Vascular System of Echinoderms. (a) General arrangement of the water-vascular system. (b) Cross section of an arm, showing the radial canal, amputate and tube feet of the water-vascular system. (n) Stepping cycle of a single tube foot. For simplification the retractor muscles in the tube foot are not shown.

(iv)    Walking: Invertebrates, such as insects, chelicerata, crustacean and uniramia with denser (than air in which they live) bodies have evolved structural support in the form of rigid skeleton that interact with ground. These structures include flexible joints, tendons, and muscles that attach to a rigid cuticle and form limbs. The limb joints allow extension and flexion of the limb. The limb plane at the basal joint with the body can also rotate, which is responsible for forward movement.

(v)     Jumping: Some insects (fleas, grasshoppers, leaf hoppers) can jump. To jump, an insect must exert a force against the ground sufficient to impart a take-off velocity greater than its weight. Long legs increase the mechanical advantage of the leg extensor muscles.

(vi)    Flight: Flight is observed in pterygote insects. Insects use a direct or synchronous flight mechanism, or indirect or asynchronous flight mechanism. For details, see chapter 9, questions number 6 and 7, objective book for paper B.

31. Give an account of structure of a striated skeletal muscle of a vertebrate.

Ans. A skeletal muscle is composed of thousands of multinucleated muscle fibres (muscle cells). The entire muscle is covered by an outer sheet of fibrous connective tissue called epimysium, while each muscle cell (fibre) is wrapped in connective tissue sheath called endomysium, and groups of muscle cells are bundled by more wrappings called perimysium and each bundle of muscle cells is called fascicle. Each muscle cell is internally composed of very fine fibres called myofibrils, packed together and invested by the cell membrane, the sarcolemma. Electron micrography and biochemical analysis show that the myofilament contains two types of myofilaments: thick filaments composed of the protein myosin, and thin filaments composed of the protein, actin, these are the actual contractile proteins of the muscle. Thin filaments project towards the centre of the sarcomere and are held by a dense structure called the Z line, while thick filaments are centered in the sarcomere. The functional unit of the myofibril, the sarcomere, extends between successive Z lines.

At rest the thick and thin filaments do not overlap completely, and the area near the edge of the sarcomere where there are only thin filaments is called the I band. The A band is the broad region that corresponds to the length of the thick filaments. The thin filaments do not extend completely across the sarcomere, so the H zone in the centre of the A band contains only thick and thin filaments is the key to how the sarcomere, and hence the whole muscle, contracts. Fig. 1.33.

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Figure 1.33 Structure of Skeletal muscle Tissue. (a) A skeletal muscle in the forearm consists of many muscle fibers (cells) (b) bundled inside a connective tissue ,shealth. (c)A skeletal muscle fiber contains many myofibrils, each consisting of (d) functional units called sarcomeres. (e) The characterisic striations of a sarconere are due to the arrangement of actin and myosin filaments.

Q. 32. How does the sliding-filament model explain muscle contraction?

Ans. In the 1950s the English physiologists A. F. Huxly and H. E. Huxley independently proposed the sliding filament model to explain striated muscle contraction. According to this model, the thick and thin filaments become linked together by molecular cross bridges, which act as levers to pull the filaments past each other. During contraction, cross brides on the thick filaments swing rapidly back and forth, alternately attaching to and releasing from special receptor sites on the thin filaments, and drawing thin filaments past thick in a kind of ratchet action. As contraction continues, the Z lines are pulled closer together. Thus the sarcomere shortens. Because all sarcomere units shorten together, the muscle contracts. Relaxation is a passive process. When cross bridges between the thick and thin filaments release, the sarcomeres are free to lengthen. This requires some force, which is usually supplied by antagonistic muscles or tile force of gravity. Fig. 1.34.

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(b)                                         Contracting

Figure 1.34 Sliding-Filament Model of Muscle Contraction. (a) A sarcomere in a relaxed position. (b) As the sarcomere contracts, the my‑ osin filaments form attachments called cross-bridges to the actin filaments and pull the actin filaments so that they slide past the myosin filaments. Compare the length of the. sarco‑mere in (a) to that in (b).

muscle fiber. In the vicinity of the junction, the neuron stores a chemical, acetylcholine, ‘ in minute vesicles known as synaptic vesicles. Acetylcholine is released when a -nerve impulse reaches a synapse.. This substance  is  a chemical mediator that diffuses across the narrow junction and acts on the muscle fiber membrane to generate an electrical depolarization. The depolarization spreads rapidly through the muscle fiber, causing it to contract. Thus the synapse is a special chemical bridge that couples together the electrical activities of nerve and muscle fibers. Fig. 1.35.

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Figure 1.35 Nerve-Muscle Motor Unit. A motor unit consists of one motor nerve and all the muscle fibers that it innervates. A neuromuscular junction or cleft is where the nerve fiber and muscle fiber meat.

Built into vertebrate skeletal muscle is an elaborate conduction system that serves to carry the depolarization from the myoneural junction to the densely packed filaments within the fiber. Along the surface of the sarcolemma are numerous invaginations that project as a system of tubules into the muscle fiber. This is called the T-system. The T-system is continuous with the sarcoplasmic reticulum, a system of fluid-filled channels that runs parallel to the myofilaments. The system is ideally arranged for speeding the electrical depolarization from the myoneual junction to the myofilametns within the fiber.

Excitation-Contraction Coupling: How does electrical depolarization activates the contractile machinery? In resting, unstimulated muscle, shortening dos not occur because thin tropomyosin strands surrounding the actin myofilaments lie in position that prevents the myosin heads from attaching to actin. When muscle is stimulated and the electrical depolarization arrives at the sarcoplasmic reticulum surrounding the fibrils, calcium ions are released (Figure). Some calcium binds to the control protein troponin. Tropanin immediately undergoes changes in shape that allow tropomyosin to move out of its blocking position, exposing active sites on the actin myofilaments. The myosin heads then bind to these sites, forming cross bridges between adjacent thick and thin myofilaments. This sets in motion an attach-pull-release cycle that occurs in ‘a series of steps as shown in Figure. Release of bond energy from ATP activates the myosin head, which swings 45 degrees, at he same time releasing a molecule of ADP. This is the power stroke that pulls the actin filament a distance of about 10 nm, and it comes to an end when another ATP molecule binds to the myosin head, inactivating the site. Thus each cycle requires expenditure of energy in the form of ATP.

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Figure 136 Excitation-contraction coupling in vertebrate skeletal muscle. Step 1 An action potential spretils along the sarcolemma and is conducted inward to the sarcoplasrnic reticulum by way rd T tubules (T-tubule system). Calcium ions released from the sarcoplasmic reticulum diffuse rapioly into the myofibrils and bind to troponin molecules on the actin molecule Troponin molecules are moved away from the a active sites. Step 2: Myosin cross bridges bind to the exposed active sites. Step 3. Using the energy stored in ATP, the myosin head swings toward the center of the sarcomere. Step 5: The a myosin head splits ATP, reataining the energy released as well as the ADP and the phosphate group. The cycle can now be repeated as long as calcium is present to open active sites on the actin molecules

Shortening will continue as long as nerve impulses arrive at the myoneural junction and free calcium remains available around the myofilaments. The attach­pull-release cycle can repeat again and again, 50 to 100 times per second, pulling thick and thin filaments past each other. While the distance each sarcomere can shorten is very small, this distance is multiplied by the thousands of sarcomeres lying end to end in a muscle fiber. Consequently, a strongly contracting muscle may shorten by as much as one-third its resting length. When stimulation stops, calcium is quickly pumped back into the sarcoplasmir reticulum. Troponin resumes its original configuration; torpomyosin moves hk into its blocking position on actin, and the muscle relaxes.

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Figure 136.a Model of the Calcium-Induced Changes in Troponin that allow Cross-Bridges to from between Actin and Myosin. The attachment of Ca2+ to troponin moves the troponin tropomyosin complex, which exposes a binding site on the actin The myosin cross-bridge can then attach to actin and undergo a power stroke.

 

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