Page:Natural History, Mollusca.djvu/86

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74
GASTEROPODA.

CLASS III. GASTEROPODA.

(Crawling Mollusks.)


If we examine the manner in which the common Garden Snail crawls, and especially if we look at it through a pane of glass as it glides up on the outside of the window, we shall see that the whole of the under-part of the body forms a wide fleshy disk, which is applied to the surface upon which the animal moves. Carefully watching this, we perceive that there are minute muscular movements constantly taking place over the whole disk, by means of which the animal advances with an uniform gliding progression. This disk, and its peculiar action, afford the most important character by which this Class of animals is distinguished, and hence they have been named Gasteropoda, which signifies belly-footed.

The upper surface of the body is covered with a fleshy cloak, the edges of which usually project in a greater or less degree, overlapping the foot-disk and other organs. This is called the mantle.

There is a distinct head, more or less conspicuous, according as it more or less projects from beneath the edge of the mantle; it is furnished with tentacles, varying in number from two to six, arranged in pairs on each side. These are probably delicate organs of touch, and perhaps they may be connected with other senses also. The majority of species are furnished with eyes, com-