PHYLUM CHORDATA (chorda, cord)

PHYLUM CHORDATA (chorda, cord)

Phylum Chordata has about 45,000. Its members are very successful at adapting in aquatic and terrestrial environments. Characteristics of the phylum Chordata are:

1. They are bilaterally symmetrical deuterostomes animals.

2. Four unique characteristics present at some stage in development. These are: noto hord, pharyngeal slits or pouches, dorsal tubular nerve cord, and postanl tail

3. An endostyle or thyroid gland is present in them.

4. They have complete digestive tract.

5. The ventral contractile blood vessel (heart).

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Basic characters of Chordates

(a)  Notochord (noton, the back + chorda, cord): This phylum is named after the notochord. It is a supportive rod. It extends most of the length of the animal into the tail. It is dorsal to the body cavity. It consists of a connective-tissue sheath. This sheath encloses cells. Each cell contains a large fluid-filled vacuole. This arrangement gives the notochord some turgidity. This turgidity compresses the anteroposterior axis. At the same time, the notochord is flexible. It allows lateral bending like lateral undulations of a lish during swimming. Cartilage or bone partly or entirely replaces the notochord in most adult vertebrates.

(b) Pharyngeal gill slits: Pharyngeal slits are a series of openings. These openings are present in pharyngeal region between the digestive tract and the outside of the body. In some chordates, diverticula – from the gut are present in the pharyngeal region. These diverticula do allow an open passageway to the outside. These diverticula are called pharyngeal pouches. The earliest chordates used the slits for filter feeding. Some living chordates still use them for feeding. Other chordates have developed gills in the pharyngeal pouches for gas exchange. Incomplete pharyngeal slits are developed in terrestrial vertebrates.

(c) Tubular nerve chord: The tubular nerve cord and its associated structure are responsible for chordate success. The nerve cod runs the longitudinal axis of the body. It is present just dorsal to the notochord. It also expands anteriorly as a brain. This central nervous system is associated with the development of complex systems. These systems are used for sensory perception, integration, and motor responses.

(d) Postanal tail: It is the fourth chordate characteristic. A postanal tail extends posteriorly beyond the anal opening. The notochord or vertebral column supports the tail.

SUBPHYLUM UROCHORDATA (TunIcates) (Uro, tail + chorda, cord)

Habitat: Members of the subphylum Urochordata are the tunicates or sea squirts. The ascidia is the largest class of tunicates. Their adults are sessile, solitary or colonial. The adult apendicularians and thaliaceans are planktonic. In some localities, tunicates occur in large numbers and become dominant animals.

Body form: Sessile Urochordata have saclike bodies. These attached to rocks, pilings, shi hulls, and other solid substrates. The unattached end of urochordates contains two siphons. These siphons allow sea water to circulate through the body.

(a) Oral siphon: One siphon is the oral siphon. It is the inlet for circulating water through the body. It is present opposite to the attached end of the ascidia. It also acts as mouth opening.

(b) Atrial siphon: The second siphon the atrial siphon. It is the opening for excurrent water.

Tunic: The body wall of most tunicates is covered by tunic. Tunic is connective-tissues like covering. It appears gel like. But it is quite tough. It is secreted by epidermis. It is composed of proteins, salts and cellulose. It also has some mesodermally derived tissues like blood vessels and blood cells. Rootlike extensions of tunic are called stolons. Stolons help to anchor tunicate to the substrate. It also connects individuals of a colony.

Maintenance Functions

Locomotion

Longitudinal and circular muscles are present below the body wall epithelium. They help to change the shape of the adult tunicate. They act against the elasticity of the tunic and the hydrostatic skeleton. Hydroskeleton is produced by sea water confined to internal
chaambers.

Nervous system

The nervous system of tunicates is present in the body wall. It forms a nerve plexus. It has single ganglion located on the wall of the pharynx between the oral and atrial openings. This ganglion is not vital for coordinating bodily functions. Tunicates are sensitive to many kinds of mechanical and chemical stimuli. The receptors for these
sense are distributed over the body wall. They are also present around the siphons. There are no complex sensory organs.

Nutrition

Pharynx and atrium: The urochordates have a very large pharynx and a cavity, called the atrium. Atrium surrounds the pharynx laterally and dorsally. The pharynx of tunicates starts to the oral siphon. It is continuous with the remainder of the digestive tract. The oral  margin of the pharynx has tentacles. These tentacles prevent large objects from entering the pharynx. Numerous pharyngeal slits called stigmas perforate the pharynx. Cilia associated with the stigmas. These cilia circulate water into the pharynx through the stigmas and into the surrounding atrium. Water leaves the tunicate through the atrial siphon.

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Internal Structure of a Tunicate (a) Longitudinal section. Black arrows show the path of water. (b) Cross section at the level of the arrial siphon. Small black arrows show movement of food trapped in mucus that the endostyle produces.

Digestive tract: The pharynx opens into digestive tract of adult tunicates. Digestive tract ends at the anus near the atrial siphon. Ventral ciliated groove is present in pharynx. It is called the endostyle. It forms a mucous sheath during feeding. Cilia move the mucous sheet dorsally across the pharynx. Food particles are brought into the oral siphon with incurrent water. They are trapped in the mucous sheet and passed dorsally. Food is incorporated into a string of mucus. The ciliary action moves this string into the next region of the gut tract. Digestive enzymes are secreted in the stomach. Most absorption occurs across the walls of the Intestine. Excurrent water carries digestive wastes from the anus out of the atrial siphon.

Respiration

The pharynx also functions in gas exchange. Water circulates through the tunicate and gases are exchanged.

Blood vascular system

The tunicate heart is present at the base of the pharynx. One vessel from the heart runs anteriorly under the endostyle. Another vessel runs posteriorly to the digestive organs and gonads. Blood flow through the heart is not unidirectional. Peristaltic contractions of the heart propel blood in one direction. Then the direction is reversed. The significance of this reversal is not understood. Tunicate blood plasma is colorless. It contains various kinds of amoeboid cells.


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Exertion

Ammonia diffuses into water. This water passes through the pharynx and is excreted. Additi’onally, amoeboid cells of the circulatory system accumulate uric acid. These cells trans r uric acid into the intestinal loop. Pyloric glands on the outside of the intestine also have excretory functions.

Reproduction and Development

Reproduction: Urochordates are monoecious. Gonads are located near the loop of the intestine. The genital ducts open near the atrial siphon. Gametes are shed through the atrial siphon for external fertilization. In some case, eggs are retained in the atrium for fertilization and early development. Self-fertilization occurs in some species. But mostly cross-fertilization occurs.

Development and metamorphosis: Tadpole like larva is developed during development. It has all four chordate characteristics. Metamorphosis begins after a brief free-swimming larval period. Larva does not feed during metamorphosis. The larva settles to a firm substrate. It attaches by adhesive papillae. These papillae are located below the mouth. The outer epidermis shrinks during metamorphosis. It pulls the notochord and other tail structures internally. They are organized into adult tissues. The internal structures rotate at 180o. It brings the oral siphon opposite the adhesive papillae. It also bends the digestive tract into a U shape.

SUBPHYLUM CEPHALOCHORDATA (kepholo, head + chorda, cord)

Habitat: Members of the subphylum Cephalochordata are called lancelets. Lancelets have four chordate characteristics. Therefore, they are often studied in introductory zoology courses. The cephalochordates consist of two genera. These are Branchiostoma (amphioxus) and Asymntetron. Lancelets have about 45 species. They are distributed throughout the oceans of world. They live in shallow waters that have clean sand substrates.

External features: Cephalochordates are small (up to 5 cm long). They are tadpole like animals. They are elongated and laterally flattened. They are nearly transparent. They have streamlined shape. The cephalochordates are relatively weak swimmers. They spend most of their time in a litter feeding position. They remained buried in this position and their anterior end sticking out of the sand.

Notochord and locomotion: The notochord of cephalochordates extends from the tail to the head. So they are name cephalochordates. Most of the cells of their notochord are muscle cells. It makes the notochord contractile. Both these characteristics are adaptations for burrowing. Contraction of the muscle cells compresses the fluids within. It increases the rigidity of the notochord. It gives additional support when pushing into sandy substrates. Relaxation of these muscle cells increases flexibility for swimming.

Muscle cells on the side of the notochord cause undulations. It propels the cephalochordate through the water. Longitudinal, ventrolateral folds of the body wall help to stabilize cephalochordates during swimming. A median dorsal fin and a caudal fin also help in swimming.

Oral hood, pharynx and atrium

(a) Oral hood: An oral hood projects from the anterior end of cephalochordates. Ciliated fingerlike projections hang from the ventral side of the oral hood. These are called cirri. Cirri are used in feeding.

(b) Pharynx and slits: The posterior wall of the oral hood have mouth opening. Mouth opens into a large pharynx. Numerous pairs of pharyngeal slits are present in pharynx. These are supported by cartilaginous gill bars.

(c)  Atrium: Large folds of the body wall extend ventrally around the pharynx. These folds fuse at the ventral midline of the body. It forms atrium. Atrium is a chamber.  It surrounds the pharyngeal region o the body. Atrium protects the delicate, filtering urfaces of the pharynx from bottom sediments. The opening from the atrium to the utside is called the atriopore.

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Maintenance Functions

Nutrition

1. Filter feeding: Cephalochordates are filter feeders. They bury in sandy substrates during feeding. Their pointed end is placed upward. Cilia are present on the lateral surfaces of gill bars. They sweep water into the mouth. Water passes from the phyarynx through pharyngeal slits to the atrium. It moves out of the body through atriopore. Larger materials catch on cilia of the cirri. The contractions of the cirri ow them out. Smaller, edible parts are pulled into the mouth with water. These are elected by cilia the gill bars in mucus. This mucous is secreted by the endostyle. Endostyle is a ciliated groove. It extends longitudinally along the midventral side of pharynx.

2. Digestion in gut: Cilia move food and mucus dorsally. It forms a food cord to the gut.  A ring of cilia rotates food cord and dislodge food. Digestion is both extracellular an intracellular. A diverticulum of the gut is called midgut cecum. It extends an riorly. It ends blindly along the right side of the pharynx and secretes digestive en ymes. An anus is present at the left side of the ventral fin.

Bloodvascular and excretory system

Cephalochordates do not possess a true heart. Contraction waves in the walls of major vessel propel blood. Blood contains amoeboid cells. It bathes tissues in open spaces.

Excretory tubules are modified coelomic cells. These cells are closely associated with blood vessels. Thus there is an active transport of materials between the blood and excretory tubules.

Coelom

The coelom of cephalochordates is reduced. It is restricted to canals near the gill bars, endostyle and the gonads.

Reproduction and Development

Cephalochordates are dioecious. Gonads bulge into the atrium from the lateral body wall. Gametes are shed into the atrium. They leave the body through the atriopore. External fertilization takes place. Bilaterally symmetrical larva is formed during development. Larvae are free-swimming. It settles to the substrate before metamorphosis and become adults.

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