ANIM ALS WITH SPINY SKINS— THE ECHINODERMS 291^ 



merits. However, the major movements are accomplished by an entirely 

 new system that is found in no other phylum of animals. This is the 

 water vascular system which uses sea water under a sort of internal hy- 

 draulic pressure to accomplish its objectives. On the aboral surface of 

 the central disc of the starfish is a hard round structure called the madre- 

 porite which is the beginning of this system. The madreporite admits 

 the proper amount of sea water into the system. A little calcified canal, 

 called the stone canal, leads from the madreporite to the ring canal, from 

 which a radial canal radiates out into each of the five arms. Alongside 

 the radial canals are little ampullae which connect to tube feet which can 



Photo by Winchester 



Fig. 20.2. Close-up of the central disc of a starfish. Note the round madreporite, the 

 numerous spines, and the small pedicellariae at the base of the spines. 



be extended through the oral surface of the body. These can be seen 

 externally lying in rows in the ambulacral grooves of each arm. To 

 extend a tube foot the starfish squeezes the connecting ampulla. This 

 forces the sea water out into the tube foot and makes it much longer 

 than it was previously. On the bottom of the tube foot, in most starfish, 

 is a little suction cup which may be attached to some solid object. Once 

 this attachment is made the muscular wall of the tube foot may contract, 

 thus forcing the excess water back into the ampulla. This contraction 

 of the tube foot will pull the starfish toward the point of attachment. 

 Through cooperative action of many tube feet the animal draws itself 

 along in a rather slow and awkward fashion. Because the tube feet 

 cannot get a grip on sand, starfish are seldom found along shores with a 

 sandy bottom, but are very abundant where the shore line is rocky. 



