STARFISH, SEA UKCHIN, BRITTLE STARS 93 



being stretched its full length, each tube foot is fastened to 

 some object by the sucker at the end. Then all the tube 

 feet are contracted and the body drawn forward. By a 

 repetition of these acts the starfish progresses over the siir- 

 face upon which it lies. 



The coelome, or body cavity. — All of the large space 

 within the central disk and arms and between the ahmen- 

 tary canal and body walls is called the coelome, or body 

 cavity. It is filled with a fluid that consists mainly of sea 

 water but has numbers of amoebahke cells floating in it. 

 This ccelomic fluid is kept in motion by the cilia hning the 

 coelomic epithelium and thus constitutes a sort of circula- 

 tion. The nutritive portions of the food are probably 

 absorbed through the walls of the stomach directly into 

 this fluid and are thereby carried to different parts of the 

 body. 



How the starfish breathes. — We have already described 

 the branchiae that project through openings between the 

 ossicles on the dorsal surface. These are now supposed 

 to act hk^ gills in gathering the oxygen from the sea water 

 and are thought to aerate the coelomic fluid which, in turn, 

 probably carries fresh oxj'gen to all the organs it bathes. 



Nervous system. — Surrounding the mouth is a five- 

 angled nerve ring, from which radiate five nerves, one for 

 each arm. Each of the radial nerves runs along the bot- 

 tom of an ambulacral groove and may be seen by parting 

 the tube feet along the middle hne. At the extreme end 

 of each of the ambulacral grooves is a small, bright red spot, 

 the eye. Over each eye is an organ that resembles a tube 

 foot without a terminal sucker. These organs are called 

 tentacles and have been shown to be organs of smell. 



Life history and reproduction. — The starfish reproduces 



