WATER SNAILS. 197 
and nearly gelatinous, very broad, obtuse at its 
extremities. Its colour is greenish-yellow in the 
thin parts, but of greenish-grey in the denser 
parts. The foot is very large and truncated in 
front. The tentacles are extremely thin and 
transparent, slightly and irregularly veined with 
bright-grey. The jaw is single, composed of a 
slightly arched piece, with an indistinct median 
projection, and covered by numerous striations. 
It is not by the mantle that the shell is 
covered, but by a simple dilatation of its border, 
which is entire and very contractile, and is pushed 
over upon the shell so as to envelop it like a 
bag. The protection thus afforded to the test 1s 
nearly always complete when the animals are 
immersed. In the larger individuals, when the 
water is deep and the sun shining, the mantle- 
expansions do not quite cover the upper surface 
of the shell, but a small round, oval, or irregular 
space is left, which enables one to see the 
speckled body. If the animal be disturbed by 
touching the edges of its sac, it endeavours to 
cover entirely the shell. The faculty of extension 
of the mantle margin is a gift of Nature, to coun- 
terbalance the extreme thinness and fragility of 
the shell. 
The animals cannot live out of the water, and 
never voluntarily leave it; they are always on 
the move, especially during sunshine, but their 
