WATER SNAILS. 
201 
brown tint of tbe water. Owing to the un¬ 
favourable state of the weather, and the con¬ 
strained position I was forced to assume, it was 
impossible to observe the animal very accurately 
in the water; but although I looked for it, I was 
unable to detect the mantle which is said to 
cover the greater part of the shell. I kept them 
in the box from Friday forenoon until late 
on the following Monday evening; they were 
then transferred to a tumbler of water, with a 
little Anacharis in it, and they seemed as lively 
and fresh as if they had been but an hour 
caught. They moved about rapidly, and with a 
continuous gliding motion over the sides of the 
glass, sailing on steadily, so to speak, without 
any of the jerking mode of progression so 
common in most of the gasteropods. They 
gradually died off, one surviving a fortnight. 
During the time I had an opportunity of observ¬ 
ing them in captivity I never could discover any 
portion of the mantle expanded over the shell.” 
Genus Physa {Bubble Shells). 
The animal is provided with two very long 
slender tentacles, bearing the eyes at their bases 
internally ; the jaw is single and slightly arched; 
the shell is sinistrally spiral, thin, and polished. 
The two British species belong to two different 
