66 HOMES T7NDEB THE SEA. 



Urchin are set rows of globular projections, varying in 

 size and number according to the species. 



Upon these projections are set the calcareous spines, 

 each of which is furnished at the base with a hemi- 

 spherical cup, a very little larger than the ball to which 

 it belongs. A muscle surrounds both ball and socket, 

 serving the double purpose of fastening them together 

 and of moving the spine in any required direction. 



In the Mamillated Urchin, a figure of which is here 



Fig. 2. MAMILLATED URCHIN bTKIPI'BD OF ITS SPIN 



given, the spines are comparatively few, but of enormous 

 dimensions. Species of similar Urchins are very common 

 in a fossil state, and go by various popular names. So 

 large are the spines of several species now inhabiting 

 the tropical seas, that, when calcined, they form excellent 

 slate-pencils. 



In our common Sea Urchins, the spines are very 

 numerous, and very small, slender, and pointed. Being 

 also brittle, they must not be handled roughly, as they 



