. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. 266 ZOOLOGY Forest snail, showing the two tentacles with an eye on the end of each. From photograph by Davison. -Wfeo^^Hti-'t body, including the stomach, are covered by it. ^^â ^^^ ^jT In most gastropods the body is spirally twisted in ^^^B^^jjtKt^-^ the shell. In the garden slug, the mantle does not ^^^B^^^^^^ secrete a shell and the naked body is symmetri- ^^^^^m cal. The twisting of the body is not seen in very â HgT young snails, so that this peculiar state is believed to be something
. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. 266 ZOOLOGY Forest snail, showing the two tentacles with an eye on the end of each. From photograph by Davison. -Wfeo^^Hti-'t body, including the stomach, are covered by it. ^^â ^^^ ^jT In most gastropods the body is spirally twisted in ^^^B^^jjtKt^-^ the shell. In the garden slug, the mantle does not ^^^B^^^^^^ secrete a shell and the naked body is symmetri- ^^^^^m cal. The twisting of the body is not seen in very â HgT young snails, so that this peculiar state is believed to be something secondar}'" which has appeared as a consequence of the animal's bearing a shell. Variability of Snail Shells. â Snail shells arc very variable in shape and color markings, as may be seen if several of the same species be ex- amined carefully. Varieties of snails in mountain valleys in certain of the Hawaiian Islands are found to be quite distinct, each in its own valley. Yet it is quite certain that all the snails of these several varieties were at one time alike. Helix nemoralis, a European snail, has been introduced into this country and has multiplied so rapidly and varied so greatly that in an area one thousand feet in diameter three hundred and eighty-five varieties have been collected, each slightly differing from the other either in color or form of shell.^ Feeding Habits. â The mouth of the snail is easily found on the under side of the foot. Just within the mouth is the lingual rib- bon. The ribbon consists of a flap of membrane bearing many sharp, filelike teeth, microscopic in size. This structure, which is moved by muscles, passes over a pad of cartilage and rubs, filelike, against the surface to which it is applied. In this manner some snails can bore circular holes in shells of other mol- lusks, in order to get the soft part, which they use as food. An example of a univalve moUusk which thus obtains food is the oyster drill, which annually does thousands
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