STARFISH, SEA URCHIN, BRITTLE STARS 93 



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

 some oljject by the sucker at the end. Then aU the tube 

 feet are contracted and the body drawn forward. By a 

 repetition of these acts the starfish progresses over the sm- 

 face upon which it lies. 



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

 within the central disk and arms and l^etween the alimen- 

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

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

 water but has numbers of amceljalike cells floating in it. 

 This coclomic fluid is kept m motion Ijy the cilia lining the 

 ccelonnc epithelium and thus constitutes a sort of circula- 

 tion. The nutritive porti(.)ns of the food are probaljly 

 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 like gills in gathering the ox}'gen from the sea water 

 and are thought to aerate the cadomic fliud which, in turn, 

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



Nervous system. — Surrounding the mouth is a five- 

 angled nerve rmg, 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 line. At the extreme enrl 

 of each of the ambulaci-al grooves is a small, liright red spot, 

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

 foot mthout a terminal sucker. These oi'gans are called 

 tenlndes and have been shown to he organs of smell. 



Life history and reproduction. — The starfish reproduces 



