WATER SNAILS. 185 
drought they partially bury themselves in the 
mud. The pond snails make a very audible 
squeaking noise on being taken out of the water. 
This arises from the expulsion of the water as 
the animal retreats within its shell. 
The food of the pond snails is animal and 
vegetable matters in different states of putridity ; 
they also feed on living aquatic plants, and the 
green confervee encrusting their shells have been 
observed to be objects of attraction among them- 
selves. Dr. Bland noticed that the water snails, 
by cleaning off the algal growth of the shells of 
their neighbours, removed the epidermis, or 
even made holes in them by this continued 
rasping ; and thereby accounted for the decolla- 
tion of the upper whorls of their shells, when 
not attributable to chemical agencies ; formerly 
this propensity was regarded as one of true can- 
nibalism, because, in the absence of other food, 
the snails devoured each other by piercing the 
shell at the apex, and eating away the upper 
parts of its inhabitant. 
The water snails are very important elements 
in an aquarium, where the removal of decaying 
vegetable matter is necessary; they cannot, 
however, keep in check the confervoid growth. 
Hight species of the genus are known in Great 
Britain, and are contained in three sections :— 
