Cases 99–101.
The Limnæidæ are only found in fresh water. Most of them occasionally rise to the surface to breathe, where they glide along foot uppermost, at times suspending themselves by a glutinous thread, after the fashion of a spider. All countries appear to have their peculiar species.
The freshwater Limpets (Ancylus) live attached to stones and leaves of plants, and have not the habit of floating, but, like the rest of the Limnæidæ, feed on freshwater algæ, confervæ, and decayed vegetable matter.
Fig. 22.
British Pond-Snail (Limnæa stagnalis).
1. Upper view: a, foot; b, tentacles; c, eye; d, muzzle.
2. Lower view: letters a, b, c as above; e, mouth; f, respiratory orifice.
(Stylommatophora.)
Cases 102–135.
True Snails (Helicidæ, etc.) have a distinct head furnished with eyes, tentacles, cutting upper jaws, and rasping teeth, and all are protected by a spiral shell. They are almost exclusively vegetable-feeders, subsisting chiefly on leaves. The sexes are not distinct. Many of the species are beautiful objects on account of the brilliancy of their colouration, and some are remarkable for the variation they exhibit in this respect. Species of Helicidæ are found in nearly every part of the world and in all situations, from sea-level to an altitude of 12,000 feet. They are fond of moisture, and in hot and dry weather retire within their shells, remaining torpid until the return of dew and rain. Helix pomatia (Case 119), which is found on the chalk in the south of England and on the Continent, is commonly eaten in Austria, France, and Belgium.
The eggs of Land-Snails vary in texture, size, and in numbers: they are usually white, but in some instances yellow and pale green. Those of some of the large South-American forms are as hard as that of a hen, and more than an inch in length (Case 120).
