T^he Shell and its Fonnation 



35 



species that adhere to rocks in shallow water 

 (Top, Purple, Winkle, Limpet), and are therefore 

 subject to the rushing of water and the heavy 

 beating of breakers, have their shells broad, smooth, 

 and free from sculpturing that might catch the 

 water and so result in the moUusk being swept 

 from its hold. Closely related species that live on 

 sandy or gravelly bottoms appear in many cases to 

 find an advantage in angles and knobs ; probably 

 because, their foothold being precarious, when they 

 are detached by waves their irregular surface 

 prevents their being swept far from the spot. This 

 difference is well illustrated by comparing the 



Common Top 



with a closely 



allied species, 



the Painted Top, 



which occurs on 



sand, and has 



the surface of 

 the shell broken up into a series of little knobs. 

 The Netted Dog-whelk lives on sandy shores, and 

 its shell has a similar surface, whilst the upper 

 surface of the animal is dotted with dark spots that 

 closely resemble grains of sand. The Common 

 Wentletrap has a great number of flat plates 

 standing out edgewise from each coil of the long 

 shell. The very ruo-ored character of the Stino- 

 Winkle's house is obviously determined by similar 

 reasons for utility ; and, though nuich less rugged, 

 the strong sculpturing of the Pelican's-foot is clearly 

 of the same order. 



How the character of the shell is modified by 



Painted Top 



Netted Dog-whelk 



