274 



ECHINODEEMATA. 



fluid into the feet and cause them to swell, and hence to project. 

 A number of feet so distended affix themselves by means of their 

 sucking discs ; they then contract and draw the body slowly in 

 the direction of the radii. The number and distribution of these 

 appendages are subject to numerous modifications. Sometimes 



FIG. 217. Sea-urchin divided along the equatorial line (after Tiedemann). D, Digestive 

 canal fixed to the shell by mesentery ; G, generative organs ; J, inter-radial plates. 



they are arranged in rows along the whole length of the meridian 

 from the oral region to the periproct (Cidaridea and Pentacta}. 

 Sometimes they are scattered irregularly over the whole surface 

 of the body, or only over the foot-like ventral surface, as in the 



PIG. 218. Longitudinal section through the arm and disc of Solust-er endeca (somewhat 

 altered after G. O. Sars). 0, mouth leading into the wide stomach ; A, anus ; L, radia- 

 hepatic diverticulum of the stomach ; (?, genital organs ; Md, madreporic plate ; Js, inter- 

 radial diverticulum of the rectum ; Af, ambulacral feet 



Holothurians. . In some cases they are confined to the oral surface, 

 as in all the Asteroidea. We are able therefore to distinguish an 

 ambulacra! and an antambulacral zone the first coinciding with the 

 oral and ventral surfaces, the latter with the dorsal surface. Never- 

 theless the ambulacral feet are variously constructed, and do not in 



