PHYLUM ECHINODERMATA

E c h i n o d e r m a t aEchinoderms includes the invertebrates.

Echinodermata include common seashore animals such as sea stars (also known as "starfish"), sand dollars and sea urchins, along with hundreds of more exotic forms. Their basic body plan is very different from other animals, but their closest living relatives are the Phylum Chordata (which includes the vertebrates).

Characteristic of Echinoderms

Free living exclusively marine forms.

Adults are radially symmetrical while larvae are bilaterally symmetrical.

Body is represented by a central disc covered by ossicles with spines called pedicellaria.

Disc may bear extensions called arms.

Digestive system is complete.

A unique ambulacral or water vascular system is present.

Nervous system has a central nerve ring with five radiating nerves.

Reproduction is sexual. Sexes are separate. Development is indirect.

Show very high power of regeneration.

Tube feet are for feeding as well.

Classification of Echinoderms

Classificationof

Echinoderms

Asteroidea

Ophiuroidea

Echinoidea

Holothuroidea

Crinoidea

These are the typical sea stars found along the seashore. They generally have hollow arms, into which the coelomic cavity extends; the radial canals are located on the outside of the skeleton. Asteroids crawl by concerted actions of their podia, and so are very sluggish movers.

The ophiuroids are active crawlers with whip-like arms that wriggle like snakes (hence they are sometimes called serpent stars). They have a robust central disc that contains all the viscera. The arms are not hollow, but are filled with a series of articulating ossicles resembling the vertebrate backbone. Ophiuroids are also called brittle stars because of their habit of releasing arms at ossicle sutures to escape predation or other antagonistic behaviors. This process of releasing a limb is called autotomy. The lost limb is eventually regenerated.

Class Asteroidea

Class Ophiuroidea

This group includes the sea urchins (regular echinoids), heart urchins (spatangoids), and sand dollars (clypeasteroids). Echinoids are usually globular, discoidal or heart-shaped and have a skeleton made of many calcitic plates. The skeleton has five ambulacral areas, each numerously perforated for the tube feet, and five interambulacral areas, which bear spines.

These are the sea cucumbers, which do not superficially resemble any of the other echinoderms. Close examination however reveals that they do have a pentaradial symmetry, but the anus is opposite the mouth on an elongated oral-aboral axis. The calcitic plates are reduced to dermal, microscopic sclerites, which are often used in classification schemes.

Class Echinoidea

Class Holothuroidea

The typical crinoid has a long stem with "roots" or some other attachment device (holdfast) at the lower end, and a cup-shaped thecum at the top. Several arms extend from the dorsal surface of the theca to collect suspended food. These arms are commonly branched and bear calcitic columnals very similar to those of blastoids and cystoids. Some crinoids have lost the stem and become mobile. Recent crinoids are found mostly at bathyal depths.

The Benefits of Echinoderms For Life• Chips from the sea cucumber• Sea urchin eggs are widely consumed in Japan• As a cleaner beach• Biologists often use sea urchin gametes and embryos for research materialThe Loss of Echinoderms For Life• Spines of sea urchins which are toxic can cause death if treated quickly

Class Crinoidea

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