Phylum echinodermata

Phylum echinodermata:

The echinoderms are mostly marine animals. They include the sea stars, sea lilies, feather stars, star fishes, sea cucumber, sea urchin .some live in oceans and at great depth.

      Echinoderms are the most abundant group of animals that are available on the deep sea floor. Most of  them are slow moving animals and most use the tube feet(podia) for attachment in the bottom of the  sea.

      Although their free living larval stages are bilaterally symmetrical, most adult echinoderms show a basic five -point (pentamerous) radial symmetry. As radially symmetrical animals ,they lack cephalization. Thus adult echinoderms generally do not have anterior and posterior ends. Instead, the body surfaces are designated as being with either oral or aboral.

They have an internal skeleton compose largely of calcium carbonate. Typically, this skeleton bears projecting spines which give the body surfaces a spiny appearance.

       Their most distinctive feature is the WATER VASCULAR SYSTEM which consists of a series of fluid -filled canals. During  locomotion, the water vascular Canal operates like a hydraulic system. Echinoderms lack definite excretory system.

Characteristics of the   echinodermata

* Most echinoderms are typically pentamerous, that is the body is usually with five -radiating areas.

* They have skeleton  made of calcareous materials. The exoskeleton carries spines on them.

* The digestive tract is complete.

*Most echinoderms are dioecious (sex are separate) and fertilization is commonly external.

Classification of the phylum echinodermata

There are quite  a huge number of classes but five (5) classes of existing echinoderms are usually recognized

* Class of Asteroidea

* Class of Ophiuroidea

* Class of Echinoidea

* Class of Holothuroidea

* Class of Crinoidea

Class of asteroidea: They are the star fishes. They possess arms and a basically star-shaped body.

Class of Ophiuroidea :They are the brittle stars and the basket stars. They lack the anus and the tube feet lack suckers.

Class Holothuroidea :They are the sea cucumber. They are soft bodied animals. They have slender bodies which are elongated at both ends.

Class of Crinoidea : The Crinoids are the most primitive. They include the sea lilies  and the feather stars.

Class of Echinoidea :These are the sea urchins and the sand dollar. They are heart-like in shape  and the tube feet end with suckers.

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