PELECYPODA. 33 



PELECYPODA*, or BIVALVES. 



The Mollusks belonging to this Class have neither head, nor true rc ageg 

 eyes, nor jaws or tongue like those of the other Classes, and are I7e 26b. 

 enclosed in a shell which consists of two plates or valves held 

 together on one side or the margin by a horny, elastic substance, 

 called the " ligament." Bivalves do not creep about in search of 

 food, but find their means of existence in the shape of minute 

 particles, both animal and vegetable, which happen to be contained 

 in the water which they breathe. Some, however, are capable of 

 locomotion by means of a well-developed foot, and a few swim 

 through the water by alternately opening and shutting their valves. 

 The body is enclosed within two lobes of the mantle, which line 

 the interior of the valves, and which at their base are firmly 

 attached to the shell, producing on the shell a scar or impression 

 called the "pallia! line." The gills are lamellar or leaf-like, and 

 placed on each side of the body. The mouth is merely an oval 

 aperture at the anterior end of the body, and generally furnished 

 on each side with soft thin flaps, or labial palps, which have the 

 function of conveying the food to the mouth. The mantle secretes 

 the substance out of which the shell is formed. The two valves are 

 always in contact at the hinge, which is generally formed by small 

 interlocking projections or hinge-teeth; and closed by large adductor 

 muscles, which are attached to impressions in the interior of the 

 shell. When these muscles cease to act, as after death, the valves 

 of the shell open in consequence of the elasticity of the ligament 

 on the dorsal margin. The majority of species have two principal 

 adductors, one at each end, like the Venus-shells, Cockles, llazor- 

 shells, &c. ; but in Oysters, Scallops, and a few others there is 

 but a single central muscle. All Bivalves are aquatic, and the 

 majority marine. They are found burrowing in sand or attached 

 to rocks. Some perforate stones and corals, others wood, and a 

 few construct a sort of nest of fragments of shells, stones, &c. 



Bivalves may be divided into two sections, the Siphonida and 



* From the Greek : pelekus, signifying a hatchet, and jwus a foot ; the foot 

 of some Bivalves somewhat resembling a hatchet in form. 



