SHELL OF ECHINIDA. 



447 



equable enlargement of the whole is provided for, and room is 

 also made for the addition of new plates around the superior 

 orifice. From the recent observations of Agassiz it appears that 

 these new plates are developed according to a spiral arrange- 

 ment ; that is, a whole circle is not formed simultaneously ; but 

 the new plates at the top of the several rows are added in suc- 

 cession. This is an interesting fact, as showing a striking 

 analogy to the growth of the leaves, and parts of the flower, in 

 Plants. (See 987, and VEGET. PHYSIOL. 302.) 



1003. The shell is covered in the living state by a membrane, 

 which is sometimes very delicate, sometimes thick and spongy. 

 This unites the bases of all the spines, and communicates with 

 the interior of the shell by delicate processes, which pass between 

 the adjacent edges of the plates ; it is by these processes that 

 the new material is secreted, which adds to the dimensions of 

 each, and thus increases the capacity of the shell. Upon looking 

 to the part of the shell which surrounds the superior orifice, it 

 is seen that the plates are there less fully developed than they 

 are below ; their edges are not in such firm contact ; and the 

 tubercles to which the spines are attached have often scarcely 



FIG. 604. SHELL OF ECHINUS, OR SKA-URCHIN ; on the right side, covered with spines; on 

 the left the spines removed. 



le their appearance. The enveloping membrane at this part, 



