In the fresh-water species the shell is not often rounded like that of the land snails, but is more frequently long and pointed, the spire resembling a church steeple. The animal, too, differs very greatly, the tentacles being either flat and triangular or long and very tapering. The eyes are not placed at the end of the eye-peduncles, as in the land shells, but are generally situated on little swellings at the base of the tentacles. They may be found in almost any body of water, adhering to stones, sticks, and other submerged objects, or crawling over the sandy or muddy bottom.
Our fresh-water snails may be divided into two classes; first, those which breathe by means of a lung and which must come to the surface at regular intervals to take in a supply of air, and, second, those which breathe by means of plume-like gills which take the oxygen directly from the water.
One of the most common and best known of the first class is the Limnaeidae, comprising the pond snails. These animals have generally a long, graceful shell, horn-colored for the most part, but sometimes greenish without and reddish within the aperture. The animal has a broad, flat foot, an auriculate or eared head, and flat, triangular tentacles. The habits of these animals are very interesting. They will wander about the sides of an aquarium, eating the growths of green scum which have collected. At this time the mouth may be seen to open, exposing the radula and the operation of eating is not unlike the motions of a cat lapping milk. They are such voracious eaters that the dirtiest aquarium will be cleansed by them in a very short time. It is interesting to note that the young animals breathe air through the water for a long time, and finally acquire the normal characteristic of the family of breathing the air directly. While submerged, the mantle chamber containing the “lung” is tightly closed so that no water can possibly get in. It is thought by some that the species of Limnaea living at great depths retain the early habit of allowing the water to fill the mantle cavity and so breathe oxygen through the water and are not, therefore, compelled to come to the surface for air.
Limnaea lives under many varying conditions, being found in the arctic regions of Greenland and Iceland as well as in the tropics, in thermal springs and those containing mineral matter, as sulphur, as well as in brackish and fresh water. They have been found at a height of over fourteen thousand feet in Thibet and at a depth of eight hundred feet in Lake Geneva, Switzerland. During times of drought when the streams are dried up and the surface of the mud is sun-cracked, the species of this family bury themselves deeply in the mud and cover the aperture with an epiphragm, in much the same manner as the land shells. This fact accounts for the apparent disappearance of all life from a pond when it dries up, and its sudden and seemingly unaccountable reappearance when the pond is again filled with water.
A genus of pond snails closely allied to Limnaea, but having discoidal or spiral shells, is Planorbis, the flat-orb shells. Instead of dragging their shells after them, as in the last genus, they carry them perfectly perpendicular, or perhaps tilted a little to one side. The animals are very rapid in movement, more so than Limnaea, which are rather sluggish. They delight in gliding rapidly about, their long, filiform tentacles waving about like a whip in the hands of an impatient driver.
LAND AND RIVER SHELLS.
FROM COL. CHI. ACAD. SCIENCES.
Top row: Physa gyrina (U. S.) Melania tetrica (Viti Islands.) Angitrema verrucosa (U. S.) Second row: Planorbis trivolvis (U. S.) Third row: Pleurocera elevatum (U. S.) Ampullaria depressa (U. S.) Limnaea stagnalis (U. S.) Bottom row: Vivipara contectoides (U. S.) Campeloma subsolidum (U. S.) Limnaea megasoma (U. S.)
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 glairy, transparent masses of jelly-like matter which are seen attached to stones and the under side of sticks, are the eggs of Physa or Limnaea.
Not all of the fresh-water 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.