ii 4 T HE LIFE OF THE MOLLUSC A 



noted colony that dwell on the sand-hills just above 

 high-water mark, which is the normal habitat of the 

 species. On the other hand, the little black and 

 white banded Snails (Helicella virgata), so common 

 over our chalk downs, when they take up their abode 

 among the sand-dunes by the sea, become dwarfed 

 in their turn. 



Marine coastal species will also show variation 

 according to the favourable nature or the reverse of 

 their surroundings. 



Marine or freshwater species compelled to live in 

 brackish-water conditions are very liable to dwarfing 

 and distortion. 



Freshwater Mussels (Unto, Anodonta) are readily 

 affected by the quality of their surroundings, while 

 Semper has shown by experiment that the volume of 

 water allowed per individual has a very marked effect 

 on specimens of Limncea stagnaUs, the size attained 

 varying directly in proportion to the volume of water 

 allowed. 



The foot largely modifies in response to individual 

 requirement, whether for locomotory or other 

 purpose. 



The Chitons and Gastropods creep about on the 

 sole of this muscular organ, the action of which is 

 best studied by observing a Snail crawling on a piece 

 of glass (Plate XXXL). The optical effect of the 

 motion suggests a rapidly flowing stream proceeding 

 from the tail towards the head, and this is due 



