WATER SNAILS. 213 
summer days may be seen floating in the water. 
When the ditches and ponds which it inhabits are 
dried up inthesummer, the animalcloses the mouth 
of its shell with a pellucid pellicle, and retires 
into the interior ofits coil, and there remains in a 
state of torpidity until the ditches are again filled 
with water. This whitish filmy covering is analo- 
gous to the epiphragm of the land-snails, and is 
similarly pierced by a minute orifice, for the 
access of air for the purpose of respiration. Te 
observe this epiphragm-like protection, take 
specimens of the P. corneus or any other species— 
P. corneus being the larger, the pellicle is more 
conspicuous—and place them in a dampish box, 
or a botanical box with a few moist aquatic 
plants ; at the end of two or three days all the 
animals will be found to have taken this pre- 
cautionary measure. 
P. corneus is frequent in the ditches and ponds 
in the eastern counties of England; it is in 
great demand in the London markets for stocking 
aquariums; in other districts it is very rare. It 
occurs in a few localities in Ireland. 
PLANORBIS contorTus—(the Contorted Coil Shell) 
(Pl. X., fig. 108).—The shell resembles that of 
the last species in its rounded whorls, but is very 
.small, not being more than two-tenths of an inch 
in diameter; the whorls are numerous and re- 
markably compact and narrow, nearly flat above, 
