Page:Natural History, Mollusca.djvu/231

 (Bivalve Mollusks.)

needs but a glance at a Cockle or an Oyster, to perceive that it is an animal lower in the scale of existence than a Snail or a Periwinkle. The absence of anything like a head, of any distinct mouth, of jaws, or tongue, or other apparatus for selecting and seizing food, as well as of the organs of sensation, together with the limited power of locomotion, proves its inferiority; and this position is fully borne out by an investigation of its anatomy.

The first character that strikes us on looking at one of these Mollusks is, that it is enclosed, more or less completely, within a shell composed of two pieces, called valves, which commonly bear a close resemblance to each other. They are united at one portion of their outline by a hinge, which allows them to separate to a certain extent, while they can be, during the life of the animal, brought together with accuracy, and held in this position with great force.

If, now, we open the shelly valves, and examine the interior, of the common Scallop (Pecten), for instance, we see that each is lined with a delicate membrane, the edges of which meet in the same manner as those of the valves. These edges are slightly thickened, studded with coloured glands, and fringed with rows of close-set, thread-like, contractile tentacles. Proceeding now to separate