July 1, 1870.] 



HARDWICKE'S SCIENCE-GOSSIP. 



151 



the famous habit of St. Simon— the material was of 

 silk, but it could not be seen. The bristles of the 

 sea-urchin present, at their base, a small hollow 

 head, which has a compression on its lower surface, 

 thus forming a cavity which fits a tubercle on the 



penetrated with countless holes, and are affixed to 

 each other by prominences, so that, looking at the 

 spine, we only see the edges of the plates which 

 compose it. A membrane covers the whole, which 

 is furnished with vibratile cilia. 



Fig. 131. Echinus mamillatus. 



Fig. 135. An Echinus climbing up the side of an aquarium. 



carapace. Each of the prickles, notwithstanding 

 its extreme minuteness, is put in action by a sepa- 

 rate muscular apparatus. They are porous, and are 

 often grooved longitudinally, being formed of thin 

 plates, which radiate from their centres. These are 



The. shape and dimensions of these spines are 

 very variable. In certain of the echinoderms they 

 are three or four times longer than the diameter of 

 the shell ; while, in others, they are only three- 

 fourths or four-fifths of that diameter; while in 



