Case 22.
The “River-Snails” (Viviparidæ) might be termed freshwater Periwinkles, as the animals of both are very similar. The true Viviparæ are viviparous. They are rather sluggish, and found at the bottom of ponds and rivers feeding on decaying animal and vegetable matter.
Fig. 7.
The Common British River-Snail (Vivipara vivipara).
a, head; b, tentacles; c, eyes; d, foot; e, operculum.
Cases 23–25.
The Cyclophoridæ are land-shells, which, however, cannot properly be considered true lung-breathers like ordinary snails. They have not the closed lung-chamber of the Pulmonates, their eyes are placed at the base of the tentacles instead of at their tips, they have a long proboscis armed with a different rasping tongue (radula), a spiral operculum, and the sexes are distinct, whereas the true Snails are hermaphrodite.
The operculated air-breathers have been divided into many sections, chiefly on account of differences in the apertures of the shells and in the opercula. They most abound in hot countries, but a few species are met with in temperate regions.
Cases 25–26.
The “Apple-Snails” (Ampullariidæ) live in the rivers and marshes of tropical regions, and, although represented by a large number of species, exhibit comparatively slight variations in form and colour. The animal has both a pectinated gill and a lung cavity, being thus enabled to breathe either water or air.
