. Animal biology. Zoology; Biology. 220 METAZOAN PHYLA Some snails are terrestrial, in which case they breathe by taking air into the mantle cavity, the surface of which is kept moist by a mucous secretion. Other snails are pulmonate and inhabit fresh water, in which case it is necessary for them to come to the surface of the water at inter- vals to fill the mantle cavity with air. In the case of others, which live at all times submerged, water is taken into the mantle cavity, where respiration may take place through the walls of the cavity or by means of gills. The majority of snails are mari
. Animal biology. Zoology; Biology. 220 METAZOAN PHYLA Some snails are terrestrial, in which case they breathe by taking air into the mantle cavity, the surface of which is kept moist by a mucous secretion. Other snails are pulmonate and inhabit fresh water, in which case it is necessary for them to come to the surface of the water at inter- vals to fill the mantle cavity with air. In the case of others, which live at all times submerged, water is taken into the mantle cavity, where respiration may take place through the walls of the cavity or by means of gills. The majority of snails are marine. Many of them reach a con- siderable size and develop shells possessing great beauty of form and color. A snail shell may be made up of many coils in one plane, or it may. Bucccff, opf/'c nerye Cere'brcr/ Fig. 125.—Somewhat diagrammatic representation of the central portion of the nervous system of Hdix pomatia. {From Lang, " Text-hook of Comparative ;) be extended into a spire. On the other hand, the mouth of the shell may be greatly expanded and the coiling be reduced to one turn or even a part of a turn, as in the abalone. There may be no coils at all, as in the case of the limpets, which have symmetrical conical shells. Still another type of snail is one fitted for pelagic life. This has the foot broadly expanded and of the shape of a pair of wings, giving to the animals the names of butter- fly snails or sea angels. Some snails secrete an operculum which serves as a lid to the opening of the shell, being so placed on the body that when the animal is in the shell the operculum just fills the aperture. Another type of snail includes the naked snails or slugs, which may be without any shell at all or may have only a small chitinous disc occupying the same location on the body as the shell and embedded in the mantle. These are in part terrestrial forms and are often seen under objects lying on the ground in fields or gardens. There are also naked marine
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