II4 THE LIFE OF THE MOLLUSCA 
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 (Unio, 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 Limnea stagnalis, 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 XXXI.). The optical effect of the 
motion suggests a rapidly flowing stream proceeding 
from the tail towards the head, and this is due 
