WATER SNAILS. 
207 
tlie interior of the shell of a violet colour, the 
aperture slightly elongated, and the epidermis 
of a deep colour. 
The species is widely distributed through the 
length and breadth of Great Britain, and is 
abundant in brooks and in the shallow parts of 
rivers, fixed on stones or shells of fresh-water 
mussels, and rarely attached to aquatic plants. 
It is not unfrequently seen out of the water, on 
the moist surfaces of rocks by waterfalls; in such 
situations I have found the shells to be of an 
unusually small size. 
I have never been able to witness the act of 
respiration in Ancyli , which appears to be only 
very rarely necessary, for the animals remain a 
very long time beneath the water, holding them¬ 
selves with a perfect immobility to the submerged 
stones. However, it must not be supposed that 
such habits indicate any close affinity of Ancylus 
to the marine limpets (Patella); because, though 
the pond-snails frequently come to the surface 
to respire, the coil-snails do so much more 
rarely, and some Physas still less frequently 
perform the act. The fresh-water limpet was 
supposed to possess gills; and though since ascer¬ 
tained, beyond doubt, to be air-breathing, yet 
it is presumed to be capable of extracting air 
from the water, for the purpose of respiration. 
