SEA-UECHrCfS AUD SEA-CUCDMBEES. 



323 



mechanism appropriated to the movement of these 

 spines. You can hardly see this 

 to advantage in the living ani- 

 mal, but here is the entire shelly 

 box of a dead Echinus, on which, 

 while for the most part the sur- 

 face is denuded of spines, a few 

 dozen remain sufficiently attached 

 to show what I wish to demon- 

 strate, viz., the mode of articula- 

 tion. Tou observe that the whole 

 globose shell is covered with tiny 

 knobs, differing in size, and not 

 set in very resrular, or at least not 



*/ o J SPINE OF ECHINUS. 



very obvious order, but showing Segment of section. 



a tendency to run in lines from pole to pole of the 

 globe. Giving attention to one of the larger of these 

 knobs, under a lens it is seen to be a hemispherical 

 eminence on the shell, the very summit of which is 

 crowned by a tiny nipple of polished whiteness, re- 

 sembling ivory. Now if we carefully lift one. of the 

 still remaining spines from its attachment, which in its 

 present dried state is so fragile that the slightest touch 

 is sufficient for the purpose, we shall note that its base 

 rests on this tiny nipple ; and on turning it up, and 

 bringing the magnifying power to bear upon its base, 

 we see that this is excavated with a hollow, whose di- 

 mensions exactly correspond with those of the nipple. 

 It is indeed a true " ball and socket " joint, like that 

 of the human hip or shoulder, and is surrounded by a 

 capsular ligament to keep it in place, the muscles 

 which sway the spine from side to side and cause it to 

 rotate, being inserted outside the capsule. Professor 



