116 PELECYPODA, 



Class PELECYPODA. 



{Lamellihranchiata^ Conchifera, Bivalves.) 



Acephalous molliisks, or those without a head, are laterally 

 symmetrical, the organs being enclosed in a mantle, one 

 leaf or fold of which envelops each side, and is itself covered 

 and protected by a valve of the bivalve shell — which is always 

 present. They all breathe by means of gills only, and are there- 

 fore inhabitants of water, either salt or fresh. The mantle is 

 usually open on the edge, but sometimes its leaves are united 

 more or less, when the mantle is said to be closed; it is some- 

 times tubularly prolonged posteriorly into a siphon. Between 

 the lobes of the mantle lay the gills or branchiae, two on each 

 side, leaf-like and striate. The most prominent organs of the 

 body are the liver and viscera. The cerebral ganglion is situated 

 above the mouth, and communicates with the other near or dis- 

 tantly situated ganglia. The mouth is at one extremity, the 

 anus at the other; the former is provided with four small, trian- 

 gular, fleshy leaflets, the extremities of the lips, used partly as 

 tentacles. The heart is in the dorsal region ; it has but one 

 ventricle, and the circulation is simple. The foot is a somewhat 

 flesh}!' mass, atrophied in the adherent species, usually suitable for 

 digging, but rarely very useful for other locomotion. The prin- 

 cipal muscles are : (1) those controlling the movements of the 

 foot, which have their insertion upon the valve, partly near the 

 hinge, partly near the adductors, forming pedal scars; (2) the 

 adductor muscles, which, running through the mass of the 

 animal, are inserted upon the middle or sides of each valve, 

 forming adductor scars or impressions. When the adductor 

 muscle is single, it is generally centrally situated, as in the 

 oyster, and such bivalves are termed monomyary ; when double, 

 one is at either side of the valve, and such mollusks are termed 

 dimyary. The contraction of these muscles closes the valves ; 

 when relaxed, the valves open by reason of an elastic ligament 

 which joins them together at the dorsal or hinge-line. 



Mostly dioecious. No sexual union, fertilization being accom- 

 plished by the surrounding water containing the male element. 



Shell composed of two valves, but with occasionally smaller, 

 supernumerary pieces about the hinge ; this latter is either a 

 plain line, or more or less thickened internallj^, and provided 

 with interlocking teeth and fossets. Some shells, from their 

 shape, cannot be completely closed, or at least portions, front or 

 back, or both, are always gaping. 



The form of the shell, number and position of the retractors, 

 scar of the mantle-margin, the hinge, its condition as to teeth, 



