ECHINODERMS. 



275 



the ambulacra forces the fluid back into the ampullae. 

 The radial and ring canals serve to connect the various 

 parts of the ambulacral system, but the functions of the 

 madreporite and stone-canal are less certain. 



All of the echinoderms are characterized by the pres- 



FIG. 91. Ambulacral system of a starfish, a, ampullae; 06. ambulacra; c, 

 radial canal; m, madreporite; n, radial nerve; p, Polian vesicle; r, 

 ring-canal; below it the ring-nerve; s, stone-canal; t, racemose vesicle. 



ence of calcareous plates in the skin, and in all except 

 holothurians these plates are united into a more or less 

 solid skeleton covered externally with a thin layer of 

 skin. When this firm skeleton is developed various 

 regions may be recognized, the most important being 

 (1) ambulacral areas, through or between the plates of 

 which the ambulacra protrude; (2) interambulacral areas, 

 embracing a row of plates at either side of the ambulacral 

 area, and (3) adambulacral, including the rest of the sur- 

 face. The ambulacral areas mark the radii of the animal 

 and the interambulacral are interradial in position. 



In all there is more or less capacity for regeneration of 

 lost parts. Thus some of the holothurians can cast out 



