Echinoderms
are characterized by radial symmetry, several arms radiating from a
central body. The body actually consists of five equal segments,
each containing a duplicate set of various internal organs. They have no heart,
brain, nor eyes. Their mouth is situated on the underside and their anus on top.
Echinoderms have tentacle-like structures called tube feet with suction
pads situated at their extremities. These tube feet are hydraulically controlled
by a remarkable vascular system. This system supplies water through canals of
small muscular tubes to the tube feet. As the tube feet
press against a moving object, water is withdrawn from them, resulting in a
suction effect. When water returns to the canals, suction is released. The
resulting locomotion is generally very slow and is called the water vascular
system. Other characteristics of echinoderms are a calcareous endoskeleton
composed of separate plates or ossicles, deuterostomous development, complete
gut, decentralized nervous system, and a marine living environment.
| Development: Deuterostomous |
| Body cavity (between endo- and ectoderm): Coelom or a special section of it called the "perivisceral coelom" |
| Digestive System: Usually complete (incomplete in brittle stars) |
| Circulatory System: Not well-understood, but seems complete although highly-reduced |
| Excretory System: Wastes probably diffuse through the water-vascular system into the water rather passively |
| Nervous System: Nerve ring around the mouth with radial nerves from there |
| Reproductive system: Separate sexes |
| Claim to Fame: Extremely successful group with numerous distinct features -- water vascular system, tube feet, five-part symmetry, tests, pedicellariae |
Star Fish
Sea Cucumber
Sea Star
Precursor to Human Systems: Advanced nervous system
