Page:Natural History, Mollusca.djvu/140

 an examination of their structure reveals particulars in which we see an evident departure from that model. The mantle is large, projecting on all sides over the foot, which is also ample; thus a deep furrow runs all round between these parts. Within this furrow, on the right side, lies the branchial plume, resembling a series of pyramids, divided into triangular leaflets. The plume is attached to the body at its base, but its tip, which points backwards, is free. The organs of reproduction and of excretion are also situated on the same side, the former before, the latter behind the gill-plume. The head is distinct, concealed by a broad veil, which is furnished with four tubular tentacles. The mouth consists of a simple muscular proboscis, or fleshy tube, which is capable of considerable elongation and contraction; it is entirely destitute of teeth or any cutting instrument, but is, nevertheless, fully able to seize and force into the stomach such materials as are used for food. The stomach is greatly developed, and in the principal genus is divided into four cavities, of which the second is muscular, and armed with shelly teeth; the internal surface of the third is increased by longitudinal plates.

In the typical genus, there is a small thin shell enclosed within the mantle, and situated in the middle of the back, so as to cover and protect the viscera. In the curious genus Umbrella, from the Indian ocean and the Mediterranean, the shell is comparatively large, external, and shaped somewhat like that of a Limpet; but in Pleurobranchæa, there is no trace of a shell either internally or externally.