208 CURIOUS HOMES AND THEIR TENANTS. 



arises how it can be made to accommodate any in- 

 crease of size on the part of the individual it incloses ? 

 In fact, an entire hard crust would not be capable of 

 distention, and it immediately is made plain that what 

 seems elegant ornamentation on the sea urchin's shell, 

 for the sole purpose of making it look pretty, is, in 

 fact, a peculiarity of structure al>solutely indispen- 

 sable to the requirements of the creature's growth. 



As the sea urchin increases in size, continual de- 

 posits of chalky matter are made on the edges of the 

 many pieces of which the entire shell is composed. 

 These parts thus keep their shapes though their size 

 enlarges. The work is done by a delicate skin which 

 covers the whole surface of the shell and the prickles 

 or spines, and, closely joined together as they seem to 

 be, penetrates between the separate plates and pushes 

 them apart by the additional matei'ial it inserts be- 

 tween them. 



It also forms the spines, and in a wonderful man- 

 ner effects their union with the shell they cover ; each 

 spine working in a ball-and-socket joint, the ball being 

 one of the buttonlike bosses which ornament the hex- 

 agonal plates of the shell, and the socket sunken into 

 the base of the spine, and the spine movable in any 

 direction at the will of the animal. 



Among these spines, of which a full-grown sea 

 urchin carries more than twelve hundred, there are a 

 number of tiny, curiously formed pincers, each of 

 which has three prongs. With these, as vAih little 

 hands, the sea urchin lays hold of seaweeds to steady 

 himself in climbing, or in picking out whatever may 



