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Cyclads, inhabits the river Nile, and ranges from thence to Cashmere. It tenanted the estuaries and creeks of Central Europe during the Pleistocene period. It is associated, at Brammerton and Balcham, with extinct species of Rhinoceros, Hippopotamus, Mastodon, and Elephants. It is also found fossilized with living species of Cyclas and Pisidium, in the Pleistocene fresh-water marls, at Stutton, Grays, Ilford, and Erith.
I whelk, limpet, &c., and a few fresh-water snails, as the common marsh snail (Paludina vivipara), which may be studied as a type of the latter.
Shell. — All the fresh-water snails live in a single or univalve shell, having the form of a more or less conical spiral; it may be regarded as a tube
wound upon itself, eachturn of which is called a whorl or volution (a), fig. 4; the lines of junction of the whorls are called sutures (b); by the close coiling of
the whorls, a pillar of shell,
9 or columella (c), in the Fig. 4.-Section of the shell monton
he shell centre, is left, and such shells are said to be imperforated; the axis of the shell, around which the whorls are coiled, is sometimes open or umbilicated : this perforation may be a mere chink (d), or it may be filled by a shelly deposit in the adult, as in many land shells. The last turn of the shell or body whorl is usually large. The base of the shell is the end opposite to the apex (e); and the aperture is entire (f), that is, not notched or produced into a canal. The margin of the aperture is called the peristome, which in Paludina is continued all round; in the Rock Snail it becomes so in the adult; more frequently, among the shells of the next class, the peristome is incomplete, the left side of the aperture being formed by the body whorl; the right side of the aperture is formed by the outer lip, the left side by the inner, or columella lip. The outer lip is thin, not thickened or reflected, as in the majority of the land shells; but in immature shells of them it is always thin and sharp. The aperture of the shell is upon the right-hand side, when the shell is viewed in a vertical position, with the aperture directed toward you: this is the general rule, but in a few of the shells of the air-breathing snails the shell is twisted in the opposite direction : in the former case the shell is said to be dextral, and in the latter, reversed or sinistral.
Lines of colour or sculpture are termed spiral or longitudinal when they proceed from the apex around the whorls; radiating, if they extend across the whorls; and transverse when they coincide with the lines of growth. The shell is secreted by the mantle, and consists of layers of membrane strengthened by calcareous matter, and does not exhibit such an amount of diversity as that of the bivalve shells. The colours which are diffused over the shell are due to the secretion from the mantle, of pigments, which are impregnated chiefly in the epidermal matter.
The aperture of the shell is accurately closed by a horny plate attached to the back of the foot, and is called the operculum (g); it is calcareous in some other snails. As regards its mode of growth, it is concentric, that is, increasing equally all round; in the genus Valvata it is spiral, growing only on one edge, and revolving as it grows.
Animal.—The body of the animal is enclosed in the mantle, which is open in front, and through which the head and foot project; the abdomen, containing the mass of the viscera, is wound upon itself in the form of the shell. The mantle in its natural position covers the back and forms a large fold or cavity, on the left side of which lie the gills, and into which the terminations of the digestive, reproductive, and urinary organs open. When the snail (see fig. 5) is protruded from its shell, the following parts may be seen :
The Foot, by which it makes its way in the world, is a very broad, flat, expanded disk, and is
in close contact with the ventral region of the body; from which circumstance the class has
Fig. 5.-Paludina vivipara (Woodward). procured the name “Gasteropoda,” or “bellyfooted.” This organ consists of a mass of interwoven muscular fibres, and progression is due to the alternate action of a set of muscles on each side.
The Head is prolonged into a muzzle, at the extremity of which is the mouth ; from the extension of the foot in front, the animal can only feed when at rest. The head bears two long and slender tentacles or horns with the stalks bearing the eyes attached to them on the outside. The tentacles are extended to the utmost, and are gently swayed to and fro when the snail walks : by means of them the animal feels its way, and is warned of danger. An appendage may be observed on each side of the head arising from the tentacles; that on the right side is the largest. The operculum is situated on the hinder part of the foot.