BIVALVE MOLLUSKS. 219 
CLASS IV. CONCHIFERA. 
(Bivalve Mollusks.) 
It 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 to- 
gether 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-lke, 
contractile tentacles. Proceeding now to separate 
