Page:Natural History, Mollusca.djvu/122

 the surface of the water, now and then lifting and puckering up the edges of the mantle, and allowing the air to bathe the sides of the body.

This is a small group considered in the number of its component species, though some of these are of unusually large size. Its distinctive characteristics are that the gills, which are either laminated, plumose, or papillose, are arranged down the sides of the back, and that the stomach is simple. By the former character it is distinguished from the Dorididæ, the feathery gill-leaves of which are retained in several of these, but never arranged in the form of a flower around the vent. By the latter it is severed from the Eolididæ, in which the stomach sends off branching tubes on each side.

There is no proper mantle distinguished as such from the general surface of the body; but there is often an elevated ridge running down each side, along which the gill-tufts are placed. In the Scyllæa, an oceanic genus found crawling among the stems and weeds of the floating gulf-weed, there are two or three erect, square lobes of flesh, projecting from each side of the back, and on the inner side of these the small tufted gills are scattered. In Glaucus, another oceanic animal of exquisite beauty, the gills take the form of fan-like pencils of filaments, diverging from the tips of long foot-stalks.

The head, in most of the species, is protected by a veil or expansion of membrane, sometimes cleft