CLASS HOLOTHUROIDEA (holothourion, sea cucumber + cockles, in form of)
1. Habitat and size: The class Flolothuroidea has approximately 1.500 species. They are commonly called sea cucumbers. Most adults range in length between 10 and 30 cm. Sea cucumbers are found at all depths in all oceans. They crawl over hard substrates or burrow through soft substrates.
2. Body plan: Sea cucumbers have no arms. Their body is elongated along the oralaboral axis. They lie on one side. They have flattened permanent ventral side. Therefore. a secondary bilateral symmetry is developed in them.
3. Tentacles: Tube feet surrounding the mouth are elongated. These tube feet are called tentacles.
4. Body wall: Their body wall is thick and muscular. It lacks protruding spines or pedicellariae. Beneath the epidermis is the dermis. It is a thick layer of connective tissue. The ossicles are embedded in it. The ossicles of sea cucumber are microscopic. The ossicles are not involved in the determination of their shape. Larger ossicles form a calcareous ring. This ring encircles the oral end of the digestive tract. This ring acts a point o attachment for body wall muscles. There is a layer of circular muscles beneath the dell is. Longitudinal muscles are present below the circular muscles. The boiled and dried body wall of sea cucumbers is called trepang in Asian countries. It is eaten or it is added to soups for flavour and source of protein.
5. Water canal system: The madreporite of sea cucumber is internal. The water-vascular systei is tilled with coelomic fluid, the ring canal encircles the oral end of the digestive tract. t gives rise to one to ten Pollan vesicles. Five radial canals and the canals for tentae s arise from the ring canal. Radial canals and tubelet. With suction cups and ampulae run between the oral and aboral poles. The side of a sea cucumber resting on the st substrate contains three rows of tube feet. These tube feet are primarily used for attachment. The two rows of tube feet on the upper surface may be reduced in size or may be absent.
4. Locomotion: Sea cucumbers are mostly sluggish burrowers and creepers. Some sea cucumbers swim by undulation of their bodies from side to side. The tube led are six t attached to body wall ossieles. Therefore. locomotion by tube feet is inefficient. Common locomotion take place by the contractions of bodv wall muscles. It produces worm like locomotory waves. These waves pass along the length of the body.
1. Ingestion: Most sea cucumbers ingest broken organic matter with the help or their tentacles. Mucus covers the tentacles. When tentacles move through substrate, the mucous u Lips the food particles.
2. Digestion: The digestive tract consists of it stomach, a long, looped intestine, a rectum and an anus. Sea cucumbers push tentacles into the mouth. It pushes the food inward. The coelomoeytes move across the intestinal wall. They secrete enzymes for digestion.
3. Transportation: Coelomocvtes also engulf and distribute the digested food. The coelom of sea cucumbers is large. The cilia of the coelomic lining circulate the fluids throughout the body cavity. It distributes respiratory gases, wastes, and nutrients. The hemal system of sea cucumbers is well developed. They have large sinuses and a network of channels containing coelomie fluids. The primary role of hemal system is food distribution.
A pair of tubes called respirators trees is attached at the rectum. Respiratory tree forms branches throughout the body cavity. the pumping action of the rectum circulates water into these tubes. The rectum dilates and water moves through the anus into the rectum. Contraction of the rectum and anal sphincter forces water into the respiratory tree. The tubules of the tree contract and water lea e the respiratory tree. Respiratory gases and nitrogenous wastes move between the coelom and sea water through these tubules.
The nervous system of sea cucumbers is similar to other echinoid. Sea cucumber has additional nerves that supply the tentacles and pharynx. Some sea cucumbers have statocysts. Some others have complex photoreceptors.
Defense of the body
The sea cucumbers seem defenseless against predators. However, they have many defensive mechanisms:
1. Many sea cucumbers produce toxins in their body walls. .Fhis toxin discourages predators.
2. Some sea cucumbers can bring out Cuverian tubules through the anus. Cuverian tubules are the tubules of the respiratory tree. These tubules contain sticky secretions and toxins. These secretions can entangle and immobilize predators.
3. Sometimes, contraction of the both all expels the one or both respirators tree, digestive tract and gonads through the anus. This process is called evisceration. Evisceration is a defensive adaptation. It discourages predators. Then regeneration of lost part takes place.
REPRODUCTION AND DEVELOPMENT
Sea cucumbers are dioecious. Theypossess a single gonad. Gonad is located anteriorly in the coelom. It has single gonopore near the base of the tentacles. Fertilization is external. The embroyos develop in to planktonic larvae. Metamorphosis take place and adult animal settle to the substrate. In some species ,the female’s tentacles trap eggs. The eggs are transferred to the body surface alter fertilization. They are brooded there. In some species, oelomic brooding also occurs. Eggs are released into the cavity. Fertilization take place and early development occur. The young come out through a rupture n the body wall. Sea Cucumbers can also reproduce by transverse fission and regeneration of lost parts.