232 SEASIDE DIVINITY. 



Let us suppose the student to obtain a spe- 

 cimen of the egg-urchin (Echinus sphcera), a 

 species which may often be found beneath the 

 seaweeds that cover the rocks on our sea-shores. 

 The shell,, it will be seen, is globular, but some- 

 what compressed, much like the orange in shape. 

 Its structure is most interesting. This is adapted 

 to suit the growth of the animal within. The 

 shell, in fact, grows with the growth of the dweller 

 within, and it does so because, unlike the lobster 

 or crab, the urchin does not cast its shell. Now 

 this necessity for the increase of the size of the 

 house with the growth of the tenant is provided 

 for by an arrangement wonderfully complex and 

 beautiful. The shell is formed in the first place 

 of hundreds of minute portions, for were the 

 panoply to consist of one piece, it would not 

 admit of increase or growth in every direction. 

 But, as it has been said, the sphere consists of 

 hundreds of minute segments of a pentagonal 

 shape. These are fitted together like the stones 

 of an arch or dome. On the inside of these 

 segments, and also between their edges, is the 

 mantle, a thin delicate membrane. It is the 

 office of this membrane to enlarge the sea-urchin's 

 house whenever he feels himself pressed for 

 room. And how does the mantle do this ? Simply 

 by secreting carbonate of lime, and thereby adding 

 to the thickness and the superficial size of each 

 individual segment of which the shell is composed. 

 It is in this way that the shells of all bivalves 

 and molluscs are increased in size. Accordingly 



