34 Shell Life 



we need only refer to the native species of Acmctia, 

 Haliotis, CalyptrcEa, and Ancylus. For creatures 

 living in shallow water along the 

 seashores, exposed to frequent lianimer- 

 ing by the waves, such a form would 

 be highly protective, as may be proved 

 by watching the Limpets (Patella) on 

 the rocks. Adhering tightly with the 

 base of their bodies to the rock, and 

 drawing down the edges of the shell 

 by muscular contraction, the wave 

 breaks upon the shell without harming or shifting it. 

 In the case of the Limpet the shell is enlarged 

 equally all round its edge, so that it fits accurately 

 to the spot of rock the Limpet has chosen for a 

 permanent resting-place. 



Even where the simple uncoiled form of shell has 

 been departed from, there has in innumerable in- 

 stances been a retention of the general external 

 shape, that the minimum of resistance shall be offered 

 to the waves. Take either of the common species of 

 Top-shells that frequent the same situations as the 

 Limpets : although the cone is here coiled several 

 times upon an axis, the external shape is still 

 that of a cone with an almost flat 

 base. 



All our univalve shells will be 

 found to be modifications of this type, 

 in some making a low pyramid, in 

 others drawn out to a proportionately 



rr^i . T. . ,. [. Common Top 



great length. This diversity ot torm 



is, no doubt, determined by the habit of the moUusk 



and the situation it inhabits. For example, those 



