SEA-URCHINS AED SEA-CUCUMBERS ^ 



You observe that the whole globose shell is covered with 

 tiny knobs, differing in size, and not set in very regular, 

 or at least not very obvious order, but showing a ten- 

 dency 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 hemi- 

 spherical eminence on the shell, the 

 very summit of which is crowned by 

 a tiny nipple of polished whiteness, 

 resembling ivory. Now if we care- 

 fully 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 



. . SPINB OP ECHINUS. 



turning it up, and bringing the mag- (Segment of section.) 

 nifying power to bear upon its base, we see that this is 

 excavated with a hollow, whose dimensions exactly corre- 

 spond 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 Edward Forbes calculates that upon a large 

 Echinus, such as this dried specimen of E. sphcera, there 

 are more than four thousand spines, every one of which 

 has the structure, the mechanism, and the movements that 

 we have been examining. Well may he say that 4t truly 

 the skill of the Great Architect of Nature is not less dis- 

 played in the construction of a Sea-urchin than in the 

 building up of a world I" 



