The Limnaeas of which we have been speaking have mostly dextral or right handed shells, that is, have the aperture on the right side when you hold the shell in the position pictured on our plate. In the family Physidae the shell is left hand or sinistral. The members of this family have shining, horn-colored shells, more or less fusiform. The tentacles are long and filiform and are constantly moving about as in the allied genus Planorbis. The animal is very active and moves about with a steady, gliding motion. It is very interesting to watch a number of Physae in an aquarium; as they are crawling along the bottom, one will be seen to rise suddenly to the top of the water and move along with the foot applied to the surface, the shell hanging down. Again, they may be seen descending, suspended by a thin thread of mucus. When the animal rises suddenly, the branchial cavity which contains the lung is heard to open with a faint, clicking sound, which is probably due to the pressure of air in the lung being suddenly liberated. Several of the species of Physa inhabit water as cold as the freezing point and they may be frequently seen in winter gliding over the bottom of a stream or pond when the surface is frozen. The little glary, transparent masses of jellylike matter which are seen attached to stones and the under side of sticks, are the eggs of Physa or Limnaea.

Not all of the freshwater pulmonates have spiral shells. A whole family, the Ancylidae, have a conical shell formed like a rounded shield, and resembling the limpets, hence called the river limpets. They are generally quite small and live attached to the interior of dead river shells and to submerged plants and to rocks. They are very interesting, but hard to find on account of their small size and inconspicuous habitat.

The second class of mollusks or those that breathe air through the water, have a respiratory cavity instead of a lung, in which is placed a series of leaflets, arranged like the teeth of a comb in two series of lines, forming the so-called gills. The mouth, also, is placed at the end of a long rostrum, or proboscis, and not in the lower plane of the head, as in the first class. Among the most common of this class are the river snails, known as Strepomatids. There are about three hundred species in this family, and with two or three exceptions they are confined entirely to the United States in geographical distribution. The shells are more or less graceful, having long, turreted spires and small apertures. The color of the shells is generally a uniform greenish or yellowish, although some species have color bands, and the aperture is frequently tinged with purple or reddish.

The animal is very interesting in captivity. It is not very bold and will lie on the bottom of an aquarium with its head and foot half protruding from its shell, and its rostrum and tentacles slowly moving about. Frequently it may be seen moving along with its head and rostrum bent down and moving about like a hound on the scent.

A family closely allied to the last is the Melaniidae, the animals of which inhabit the entire world, except North America. They may be distinguished from the last family by the presence of little finger-like digitations on the edge of the mantle. The shells are generally larger and more highly colored than those of the last family, many of them being of a dark chocolate color and some are of a beautiful glossy black; some shells are smooth, while others are ornamented by knobs and spines. The genus Melania, a species of which is illustrated on the plate accompanying this article, is the most characteristic form.