Page:Popular Science Monthly Volume 42.djvu/203

Rh color of the animal corresponds with the prevailing color of the environment, and thus it escapes the notice of its enemies.

A more complex state of affairs exists in those animals which make use of external objects for their own concealment. A case in point is the hairy snail (Helix hirsuta), a species commonly found throughout the Northern States, living around decaying logs in the forests. These little fellows have a clothing of short hair all over the shell, and this hair holds so much of the soil that they look more like small pellets of earth than like snail-shells. The disguise is effective enough to deceive more acute shell-collectors than the birds. An altogether similar attempt at deception is practiced by a marine mollusk, the so-called "carrier." This gastropod has a broad spiral shell, to the upper surface of which it cements shells or pebbles, until finally it appears to be nothing more than a heap of shell-fragments, not distinguishable from any other irregularity of the sea-bottom. Another instance may be mentioned, as it illustrates the extension of this general principle to widely different groups of animals. The sea-urchins of our coasts have often been observed to cover themselves completely with small stones, so that nothing can be seen but a heap of pebbles.

Coming back to our hairy Helix, we may perhaps credit its hirsute coat with an additional function besides mere dirt-gathering. Poulton has observed that some insectivorous animals have an excessive repugnance for hairy insect larvæ, even when they



are not otherwise repulsive. The marmoset, for instance, can not be induced to touch any hairy larva. It is not improbable that small mammals, such as moles and field-mice, find the hairy covering of Helix hirsuta disagreeable, although we know that they eat other snails.

An old collector, who had spent most of his life within the