Page:Natural History, Mollusca.djvu/118

 themselves to deep water, and are to be obtained only by dredging.

The body in this, the principal genus, is usually of a semi-oval form sometimes very convex, but more generally depressed, and occasionally almost flat. The mantle is ample, projecting over the head, and on all sides; its surface is generally rough, with numerous warts, and its texture is stiffened with calcareous spicula or crystals of lime imbedded in its substance. The mouth is commonly without jaws, but is frequently furnished with a prehensile collar, armed with minute spines. The tongue is covered with rows of teeth curved backwards.

The breathing organs consist of flat plumes, sometimes small and simple, sometimes large and branching, either united at the base into a flower-like expansion, or placed separately in a circle more or less perfect. In some species these plumes are capable of being withdrawn into a common cavity, the margin of which can be closed completely over them, but in others there is no cavity; and when danger threatens, the plumes are merely contracted, curled up, and bent down towards the centre of the circle.