WATER SNAILS. 
203 
the inner bases of the tentacles, and are very con¬ 
spicuous, their black colour contrasting strongly 
with the yellowish grey of the body. The 
edges of the mantle are extended into about 
twelve lobes, which can be spread over the shell, 
nearly meeting above. 
This little Physa can raise itself in the water 
or sink at pleasure, without having recourse to 
plants to assist its progress; in descending 
through the water the movement is gradual and 
uninterrupted, and is effected by means of a 
thread affixed to the surface—-a mode of pro¬ 
gression analogous to that of some of the land 
slugs which I have referred to, as spinning a line 
of the gelatinous secretion from their skin; at 
other times it rises rapidly like a bubble from 
the bottom; it floats, walking along, as it were, 
immediately beneath the surface of the water by 
a series of jerks. 
Physa (Aplexa) hypnorum —(Pl. X., fig. 112). 
—The inappropriate specific term signifies fre¬ 
quenting Hypnum , a genus of the mosses. 
The shell of this species is easily distinguished 
by its elongated spire, and is half-an-inch or 
more in length, of a reddish colour. 
P. hypnorum is very gregarious and prolific; 
it inhabits ponds and ditches, especially in the 
southern and midland counties of England; it 
occurs in Scotland and Ireland, but is local. 
