Echiderms are primitive animals that have a system of internal water-filled canals, which in many echinoderms form suckered "tube feet", with which the animal may move or grip objects.

They include star fish, sea cucumbers and other marine animals.

GENERAL CHARACTERISTICS OF ANIMALS IN THE PHYLUM ECHINODERMATA

• Around 6000 species: all are marine animals.
• Most occur in relatively shallow water close to shore.
• All have a water vascular system –a series of coelomic tubes filled with water.
• Most have two sexes (rarely hermaphrodite).
• Tube feet appendages are highly characteristic of the phylum.
• The gut structure can vary: it might or might not have an anus; generally there are no special excretory organs.
• The body is usually pent radially symmetrical (ie. when cut in half, it forms five similar sections around a central point as seen clearly with a star fish).
• The nervous system is well developed but mostly over the epidermis or outer layer.
• Special sensory organs are present, but poorly developed.
• The epidermis is ciliated, and covers a calcareous endoskeleton which can often grow out to produce spines, sometimes large.
• Eggs develop inside the body and are born as miniature adults (free-swimming larvae).
• Usually, but not always, the eggs receive no nutrition or metabolic support from the parent, while developing inside the parent’s body.
• There have been 16 classes, though all but six classes of Echinoderms are now extinct.

All animals in the phylum Echinodermata live exclusively in the ocean. There are around 6000 species in the phylum, including starfish, sea cucumbers, sea urchins, brittle stars and crinoids.

The adults have a unique radial symmetry, with structures often repeated in multiples of five; eg. the arms of the starfish. They have a leathery skin which surrounds an internal skeleton of interlocking calcium carbonate plates, known as ossicles.

Echinoderms have a unique water vascular system – a network of canals, pressure relief valves and tubular suckers – which enables their movement and feeding. As water moves across a dorsal sieve plate – the madreporite – water pressure or suction is created in various parts of the vascular system. Thousands of tube feet apply suction and secrete mucus. On each tube foot is an ampulla, a small balloon-like structure which contracts and expands, forcing fluid to expand the tube foot, enabling the animal to pull itself along or feed.
 
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