The history of the inner lung is indicated by fishes and amphibians. The history of the outer lung is indicated in these naked mollusks.
Eolis, which shows us the beginning of a liver, or perhaps the last stage of its reduction, seems to be prehistoric as to the gill. One part of the surface absorbs oxygen as well as another. If we leave the beach and the Eolids for mid-ocean and the Pteropods, we shall find the first shadowing forth of a gill. In the Pteropod one part of the skin is a little more vascular than the rest, and on this part the blood is more freely oxidized. Now "respiratory activity," as Spencer has shown, "aids in the development of respiratory appendages." A larger and larger surface is exposed to the water, and this larger surface, developed partly by Natural Selection, and partly by respiratory activity itself, is attained in multitudinous branchings of the mimic tree, and deep sinuosities of the mimic leaf.
But in Doris, which represents a great advance in gill development over a Pteropod, the gill is still imperfect, and as a respiratory organ it is supplemented by the creeping disk. In Aplysia the gill is carried up to perfection and aerates all the blood.
In the evolution of an organ we have hints as to the evolution of a species.
No interest can attach to such low forms of life as the Eolids unless they teach something of the methods of Nature in originating species. Readers of The Popular Science Monthly will not give their attention to mere description or anecdote. Facts they know do not pass into science until fertilized by ideas. We shall return to Eolis and its family through a study of forms which the eye, not aided by the knife, would report as far removed from them.
A mollusk is a soft, fleshy, sac-like body, with a mantle (pallium) extending from the back in two folds, right and left, around the sides. In the Bryozoan (moss-animal), whose reticulated coral incrusts many shells and sea-weeds, the molluscan type reaches down almost to the polyp. The Bryozoan has a cylindrical body with a tentacular crown. Structurally it is a mollusk, morphologically a polyp. It would seem to be a case in the organic world analogous to that in the inorganic, in which a small portion of a mineral, in crystallizing, forces a large portion of a foreign mineral into its own crystalline form and masks the structure under the shape.
The mantle performs important functions, and it will guide us along a series of transformations. Suppose that the two folds cohere along their edges. The mantle would then become a kind of sac, inclosing the body. If we call it a tunic, we might say that the animal is wrapped in its tunic, and this cohering of the tunic-folds would bring us to the order of Tunicata.
If we put the dredge down fathoms deep into the sea, it may bring from the bottom a Clavelina, most beautiful of Tunicates. In shape it is a pitcher without handle, an inch high, tapering down to a slender
