THE IMPENETRABLE SEA 



little and shell material deposited, forming a larger 

 extension of the first shell, which was only a tiny trans- 

 parent bead. So the building process went on, each 

 extension of the shell being larger and generally thicker 

 as more and more shell material was laid down. 



The varied shapes of sea-shells are all built in this way : 

 the creatures within them having their own inherited 

 ''blue-prints" of the houses they patiently build. Molluscs 

 with heads, like the periwinkle, the snail and the whelk, 

 have their mantle all in one piece and so grow single or 

 univalve shells. The oyster (like other headless molluscs) 

 lays down separate shells with each half of its mantle, 

 so that it becomes a bivalve. 



The periwinkle, like the whelk, has its own rasp, which 

 it uses to scrape pieces off the seaweed it passes over, 

 leaving tiny indentations in the weed as it goes. The rasp 

 wears away, but the periwinkle has its own method of 

 replacing the worn section. If you could look into its 

 mouth you would find the lower part paved with sharp 

 teeth, as though a number of tiny nails had been driven 

 into it, with their points outwards. This rasp — sometimes 

 called a tongue, though radula is the more appropriate 

 name — is only the end of a strap (often two and a half 

 inches long) furnished with six hundred rows of teeth, 

 three in each row, which is coiled up in a fold of the 

 periwinkle's body. Its edges are folded together at the 

 back of the animal's mouth, and from that point it goes 

 backward, folded, as a reserve of gristly, spiked strap. 

 As the front portion becomes worn so it is broken off and 

 the back portion feeds in to the periwinkle's mouth, 

 furnished with new teeth, ready-sharpened for use. 



Shelled sea creatures may be said to live within their 

 skeletons : the hard parts of their bodies which form a 

 protecting armour, properly described as an exoskeleton. 

 But there are many different kinds of structure and 

 material used by Nature to support and protect the softer 

 and more vulnerable parts of inhabitants of the sea, and 



136 



