Page:Popular Science Monthly Volume 8.djvu/201

Rh rich to be limited to one or two efficient causes—carried it to the order of Gasteropods.

In this order we find Eolis, and Doris, and Aplysia. From them our studies have ranged over kindred, near and remote. From their kindred we return, prepared by what we have found to interpret them. In form, these animals do not depart from bilateral symmetry, as from their habits they should not. Each side is exposed in the same way to the same environing element. But the alimentary canal is bent out of line with the axis of the body. The reproductive system is still more askance. It is altogether one-sided. Very suggestive facts. We find one-sided growth without the conditions which induce it. These conditions must have pertained to an ancestor. The bend in the alimentary canal and the displacement of the reproductive organs have been inherited from an ancestor so conditioned in the environment as to produce overgrowth of one side. But the alimentary canal does not bend out of line so much as in the shell-bearing Gasteropods; and in Eolis—in which the last vestige of a shell has disappeared—the canal has become straight. Another suggestive fact. We find in these naked mollusks heredity and abbreviated heredity. Aplysia and Doris inherit the ancestral twist. In Eolis the heritage is cut off.

From symmetry to asymmetry, from a bivalve to a univalve, Nature has moved, closing a cycle of evolution in the snail; from asymmetry back to symmetry, from a shell-bearer to a non-shell-bearer, she is moving in the sea-slugs. In this retrogression, Aplysia has shared the least. It retains the largest shell-vestige; it has the most perfect liver; its gills cover the mantle. Eolis has been carried back the farthest. In this retrogressive movement we may find the rationale of Aplysia's many stomachs, and Eolis's branching stomach and hepatic cells. In the snail, perhaps in all Gasteropods, the alimentary