4i6 



ANIMAL LIFE-HISTORIES 



place in the defences. But quite a number of sea-snails, and a few 

 freshwater ones, have evolved a different type of shell, correlated 

 with a creeping life on rocks, &c., in pursuit of vegetable food, 

 and sometimes associated with a permanent home. This is the 

 case with all those forms known as " limpets ", in which the 

 shell is cap-shaped or conical, sometimes with a small spiral at 

 its tip. There can be no doubt that the ancestors of such 

 species possessed the usual spiral shell, which may indeed be 

 present in early life, recapitulating the former state of things. 



Fig. 941. — Purple-Shell {Ptirpura lapUlus) 



The limpet-shaped shell is correlated with the presence of an 

 unusually large oval foot, and when danger threatens this holds 

 on to the underlying surface with great tenacity, and the shell 

 is pulled down over the body. The two methods of defence 

 just indicated may be seen side by side on 'tween-tide rocks. 

 The Purple- Shell [Purptira lapillus, fig. 941) possesses an un- 

 usually thick shell and a strong operculum. When withdrawn 

 into its dwelling it can defy most enemies, and stand a good deal 

 of knocking about by the waves. The Common Limpet {^Patella 

 vulgaia, fig. 942), on the other hand, unless it happens to be 

 crawling over a very rough surface, is extremely difficult to 

 detach unless taken unawares. The possession of a smooth 

 place as a permanent residence is here so important that the 

 animal returns to the same spot after every feeding excursion, 



