CLASS TURBELLARIA: THE FREE LIVING FLATWORMS
Members of this class are mostly free-living. They are bottom dwellers in freshwater and marine environments. They crawl on stones, sand, or vegetation. Turbellarians are named for the turbulence. The beating of cilia creates this turbulence in the water. Turbellarians are predators and scavengers. Most turbellarians are less than 1 cm longs in the tropical areas. They may reach 60 cm in length.
There are following layers in the body wall:
- Epidermis: Its outer body layer is epidermis. Epidermis is ectodermal in origin. Epidermis is in direct contact with the environment. Some epidermal cells are ciliated. Some cells contain microvilli.
- Basement membrane: A basement membrane of connective tissues is present below the epidermis. It separates the epidermis from mesodermally derived tissues.
- Muscular layers: An outer layer of circular muscle and an inner layer of longitudinal muscles are present. These layers lie beneath the basement membrane. Some muscles are dorsoventrally and obliquely placed. These muscles are present between the dorsal and ventral side.
- Parenchyma: These are loosely organized cells. They are present between the Ion itudinal muscles and the gastrodermis.
- Gastrodermis: It is the innermost tissue layer. It is derived from the endodermis. It con ists of a single layer of cells. Gastrodermis lines the digestive cavity. The gasirodermis secretes enzymes. These enzymes help in digestion. Finally gasjrodermis absorbs the end products of digestion.
- Glands: Several types of glandular cells are present on the ventral surface of the body wall. These are epidermal origin. These are:(a) Rhabdites: These are rod like cells. They swell and form a protective mucous sheath around the body. Therefore, they protect the body from predation or desiccation (dehydration).(b) Adhesive glands: They open in the epithelial surface. They produce a sticky chemical. It attaches part of the turbellarians to a substrate.(c) Releaser glands: They secrete a chemical that dissolves the attachment as needed.
They have bilateral symmetry. Bilaterally symmetrical animals have active lifestyle. Turbellarians are primarily bottom dwellers. They glide over the substrate. They move by using cilia and muscular undulations . They spread a sheet of mucus during their movement. This mucous help in adhesion and helps the cilia gain traction. They have densely ciliated ventral surface flattened body. These adaptations help in locomotion.
DIGESTION AND NUTRITION
(a)Digestive cavity: Digestive cavity is absent some marine turbellarians. But most of them have blind digestive cavity. This cavity may be simple and unbranched chamber. Or it may be highly branched system of digestive tubes. The digestive tracts of some other turbellarians are lobed. Highly branched digestive system is an advanced structure. Gastrodermis has close contact to the sites of digestion and absorption. Therefore, it reduces the distance for the diffusion of nutrients. This branched digestive tract structure is especially important in some of the larger turbellarians. It partially compensates the absence of a circulatory system.
(b) Food: Most turbellarians are carnivores. They feed on small invertebrates. Or they scavenge on larger dead animals. Some are herbivores and feed on algae. They scrape this algae from rocks. Sensory cells are present on their heads. They help them to detect food from a large distance.
(c) Ingestion of food: The turbellarians pharynx functions as an ingestive organ. It may be a simple and ciliated tube or it may be a complex organ developed by the folding of muscle layers. In this case the free end of the tube lies in a pharyngeal sheath. It can project out of the mouth during feeding.
(d) Digestion: Food digestion is partially extracellular. Pharyngeal glands secrete enzymes. These enzymes break down food into smaller pieces. These smaller pieces are then taken into the pharynx. Phagocytic cells engulf these pieces of food from the digestive cavity. Food is broken intracellular vesicles (food vacuoles) and digestion is completed.
Fig: Protonephridial System in Turbellarians (a) Protonephridial system. (b) Numerous, fine side branches from the tubules
EXCHANGES WITH THE ENVIRONMENT
The turhellarians do not have respiratory organs. Therefore, the respiratory gases are exchanged by diffusion through the body wall. Most metabolic wastes (ammonia) are also removed by diffusion through the body wall.
Marine invertebrates are often isotonic to their environment. But they are hypertonic to their aquatic environment in freshwater. Therefore, they must regulate the osmotic concentration of their body tissues. This osmoregulation enables them to live in freshwater.
Their excretory organs are protonephridia. Protonephridia are networks of fine tubules. These tubules run the along length of the body. The branches of protonephridium end in flame cells. Flame cells have numerous cilia. These cilia project into the lumen of the tubule. A slit like openings is present in the wall of flame cell. The beating of cilia creates a negative pressure in the tubule of flame cell. Therefore, the fluid from the surrounding tissue is sucked by the tubule through flame cell. The tubules join and open to the outside of the body through a nephridiopore.
NERVOUS SYSTEM AND SENSE ORGANS
Two types of nervous systems are found in turbellarians:
1. Diffused nervous system
It is the most primitive type of flatworm nervous system. It is found in order Acoela. It composed of a sub epidermal nerve plexus (ik.). This plexus resembles the nerve net of cnidarians. A statocyst is present at the anterior end. It functions as a mechanoreceptor. It detects the position of body with respect to gravity.
2. Central nervous system
Some turbellarians have a more centralized nerve system. It composed of with cerebral ganglia. Several pairs of long nerve cords and sub-epidermal nerve net. It is found in most other turbellarians, such as the planarian (Dugesia). Lateral branches arise from the nerve cords. These lateral branches are called commissures. They connect the nerve cords. Nerve cords and their commissures give a ladder like appearance.
Neurons and sense organs
The neurons may be sensory or motor. It is an important evolutionary advancement. The neurons concentrate in the anterior end to form a pair of cerebral ganglia. It may be called a primitive brain. Turbellarians respond to a variety of stimuli in their external environment. They have following sensory cells:
(i) Many tactile and sensory cells distributed over the bodSt These cells detect touch. water currents, and chemicals.
(ii) Auricles (sensory lobes) project from the side of the head.
(iii)Chemoreceptors: They help in food search. They are especially dense in the auricles.
(iv)Eye spot (Ocelli): Most turbellarians have twr simple eyespots called ocelli (sing.. ocellus). These ocelli help the animal to detect light. Most turbellarians are negatively phototactic and move away from light. Each ocellus consists of a cuplike depression. It is lined with black pigment. Photoreceptor nerve endings are present in the cup.These are part of the neurons. They leave the eye and connect with cerebralganglia.
REPRODUCTION AND DEVELOPMENT
Many turbellarians reproduce asexually by transverse fission. Fission begins as a constriction behind the pharynx. The two (or more) animals formed as a result of fission are called Zooids. Zooids separate from each other fission and regenerate the missing parts. Sometimes, the Zooids remain attached till completion of development. Then they detach independent Individuals.
Turbellarians are monoecious. Their reproductive organs are formed from the mesodermal tissues in the parenchyma.
1. Male reproductive organs: Numerous paired testes lie along each side of the worm. Sperm ducts (vas deferens) open into seminal vesicle. Seminal vesicle is a sperm storage organ. A protrusible penis is present. The penis projects into a genital chamber.
2. Female reproductive organs: The female system has one to many pairs of ovaries. Oviducts start from the ovaries and open into the genital chamber. Genital pores open outside through genital pore.
3. Fertilization: Turbellarians are monoecious. But sperms are exchanged between two animals. This cross-fertilization produces greater genetic vesicle diversity. The penis of each individual is inserted into the copulatory sac of the partner. After copulation, sperm move from the copulatory sac to the genital chamber. They reach ovaries through oviducts.
Fertilization occurs in ovary. Yolk is directly incorporated into the egg during egg formation. Sometimes, yolk cells are attached around the zygote as it passes through yolk glands.
4. Development: Eggs are laid with or without a gel-like mass. A cocoon encloses many turbellarians eggs. Cocoon is a hard capsule. These cocoons attach to the substrate by a stalk. Cocoon contains several embryos. Two kinds of capsules are laid.
(a) Summer capsules hatch in two to three weeks and immature animals is produced.
(b) Autumn capsules have thick walls. It can resist freezing and drying. They hatch after over wintering.
Development of most turbellarians is direct. Gradual changes takes place in embryo and change it into adults. A few turbellarians produce a free-swimming Muller’s larva. It has ciliated extensions for feeding and locomotion. The larva settles on the substrate and develops into a young turbellarian.