282 



CRUSTACEA, ECHINODERMS, ETC. 



which frequent our own shores. The spines which 

 have earned for this group the popular name of " Sea- 

 urchin," are attached to minute rounded tubercles, on 

 the plan of a "ball-and-socket joint," and can there- 

 fore move about in all directions. But the ambulacral 

 or sucking-feet can be protruded even beyond them, 

 and thus the Echinus, as well as the star-fish, can 



Fig. 214. 



" Five-fingered " Star-fish (Uraster rubens}. 



glide horizontally, vertically, or even on surfaces over- 

 head, with a quiet, ghost-like motion ; all the suckers 

 being used to warp the body along. In the star-fishes 

 (with the exception of the brittle stars, which have no 

 "water-vascular system," as the mechanism of suck- 

 ing feet is called, but move about by entwining their 

 lime-plated, snake-like arms) the sucking feet are 



