Starfish

Starfish are composed of a central disc from which arms sprout which makes it a radial symmetry. They are triploblastic because like humans they have three germ layers such as the ectoderm (outside), mesoderm (middle) and endoderm (inside). The coelom of a starfish is the secondary body cavity that forms by the partitioning of three body cavities. Starfish are really unique because they do not have blood but instead use sea water to pump around their bodies. The water vascular system uses cilia and the constant contracting ampullae to keep things moving. An ionic imbalance causes water to flow into the madreporite, entering the water vascular system. Some of this water is turned into another direction into the perivisceral coelom (the large cavity), where it is circulated by the beating of cilia. Most oxygen enters the starfish through diffusion into the tube feet, or the papulae (small sacs covering the upper body surface). The starfishs mouth is located underneath the sea star on the oral or ventral surface, it eats  bivalves like mussels, clams, and oysters; or any animal slow enough to be unable to protect it self. Some have specialized tube feet that can extend and catch preys. Starfish have two stomachs including cardiac and pyloric. Cardiac stomach is pushed out of the organism's body and used to digest food. Starfishes dont have a brain, their ring nerves and radial nerves control their direction and balance. But their tube feet, spine, pedicellariae are sensitive to touch.
Crinoidea

Ophiocistioidea

Asteroidea

Echinoidea

Holothuroidea

http://www.earthlife.net/inverts/echinodermata.html

http://www.junglewalk.com/info/star-fish-information.htm