The word  echinoderm means   �spiny-skin� .  Echinoderms are invertebrates with rough, spiny skins.

Characteristics of Echinoderms

2. The internal skeleton of echinoderms is made of many small calcium carbonate plates that project out from the body to form spines.

3.Gas exchange occurs
  through their body
  surface.
4. They have no
  circulatory system

5- Echinoderms
  generally reproduce
  sexually.
  Sea stars reproduce
  by regeneration.

Regeneration:
one part of the organism is separated from the body and forms a new individual.

The Phylum Echinodermata includes the sea urchins,
sea stars, brittle stars, crinoids and
sea cucumbers.

These organism all have projections from the skin (echino:spiny,derm:skin)

The echinoderms are all aquatic organisms.

Echinoderms have a water vascular system which is used for locomotion, respiration, sensory reception, attachment to surfaces and to catch prey.

The system comprises a sieve plate,a ring canal and radial canals. Sea water enters the sieve plate and fills the ring canal.

The ring canal opens into the radial canals which branch into each arm.

The radial canals form many small tubular structures, each called a tube foot.

The tube foot has an ampulla, a muscularized water bladder at one end and a sucker at the other end.

Water is pumped into the tube foot by contraction of the muscles which surround the ampulla.

 As a result the foot elongates. The tube foot shortens when the muscles running the length of tube root contract.