Page:Popular Science Monthly Volume 42.djvu/205

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the most troublesome of these, if we may judge by the elaborate means which have been evolved for protection against them. Beetles, especially those of the family Carabidæ, are not outdone by a Provençal vintner in their appreciation of a toothsome Helix; and it is interesting to watch the celerity with which a beetle of the genus Cychrus or Dicælas eats a snail out of his shell.

PSM V42 D205 Atopa achatina.jpg
Fig. 5.-Atopa achatina. Broken, to show the narrowness of the passage between the fold.

Various organs have been modified or developed to prevent intruders from entering the aperture of the shell. In one group of land snails there is a calcareous plate, called the operculum, secreted upon the upper surface of the fleshy foot of the animal. This plate is the last part to enter the shell when the animal retracts itself, and it fits closely into the aperture, closing it like a valve or door. Examples of this structure are seen in the common Helicinas of America and the genus Cyclostoma of Europe.

The majority of air-breathing snails, however, are far removed genealogically from the operculum-bearing group, belonging, indeed, to quite a different phylum in which the operculum has never been developed. Failing this, a completely different structure has been adapted to the same use. From the rim or lip around the aperture spring processes of the shell substance, projected into the opening of the shell and decidedly narrowing it. An example of this structure in its primitive form is seen in the three-toothed snail, Helix tridentata, and its allies. The projecting "teeth," as these processes are called (although they have nothing to do with the true mouth of the animal), are prominent enough to exclude the larger beetles, and insects sufficiently small to be admitted would be drowned in the viscid mucus or slime freely exuded by the snail.

More complex is the obstructing mechanism in many of the snails of the Southern States and of South America. In some species—the Caracolus labyrinthus of Panama, for example—one wonders how the snail himself can get in or out of his own shell, so tortuous is the passage. These Southern forms represent the highest development of the three-toothed type of aperture. The meaning of this increasing complexity appears, when we remember that the regions where the most complicated types of snails are found are known to be practically coincident with the regions producing carnivorous Coleoptera in the greatest numbers and the most exuberant variety of forms. The writer once confined in a box with carnivorous beetles a number of snails with strongly "toothed" apertures (Helix uvilifera) and a number hav-