hump. Aplysia has a row of stomachs, and, what is strange, the teeth are not inserted in the mouth, but in one of the stomachs. In Aplysia, the liver is better defined than in Doris, and the leaf-like gills aerate blood for the whole body.
The classification of these naked mollusks will be as obvious now to the reader as to the observer.
In Eolis no liver, but a few bile-cells representing its rudiment, or vestige; no lung, every part of the surface respiring for itself; no well-differentiated stomach, but an arborescent intestinal tube.
Fig. 2.—Doris plumulata.
In Doris (sea-lemon), a liver; respiratory organs in the guise of crown, or star, or leaf, or tufts of sea-weed, organs which serve the liver only; a stomach.
In Aplysia (sea-hare), a better liver, respiratory organs in the form of leaves, organs which serve the whole body; many stomachs.
Eolis stands lowest, Aplysia highest. The series is suggestive of the history of organs, if not of species. It invites special attention to the lung.
In all marine animals except Cetacea, either the entire outer surface absorbs oxygen and exhales carbonic acid, or part of this surface has been differentiated for the function of respiration. In all mammals, and birds, and mature reptiles, part of an inner tissue has been differentiated and set apart for the function of respiration. External respiratory organs rise from the skin. Internal respiratory organs rise from the skin of the throat. Internal respiratory organs exist in the fish as a rudiment. External respiratory organs appear in embryotic mammals as vestiges.
The inner lung begins as a little hollow bud on the throat. This bud pushes out another and another, and so on till by continuous budding it becomes a tree-like growth, interlaced with blood-vessels. Let such a bud start from the outer surface, on the back. It will become, according to the mode of secondary budding, a little tree, or leaf, or flower of blood-vessels and vascular tissue—such a lung as adorns the back of Doris.
