392 GENERAL STRUCTURE OF LAMELLIBRANCHIATA. 



some distance from the hinge, and of which the insertion can be 

 easily traced by a somewhat rough depression or pit, on the 

 interior surface of each valve. In some Conchifera, this muscle is 

 single, and in others it is double, the two parts being even at oppo- 

 site ends of the valve (Fig. 585, c). Upon this character it has been 

 proposed to found the primary division of the Class into Orders ; 

 but the classification thus formed is not a natural one, inasmuch 

 as it brings together kinds which have little resemblance, and 

 widely separates others which are closely allied. In Fig. 585 

 are shown the several parts of a Bivalve Shell, with the expla- 

 nation of the names by which they are described. 



933. In order to describe the general structure of the Lamelli- 

 branchiata, it will be advantageous to select some particular 

 illustration ; and the Mactra (Fig. 586) is well adapted to this 

 purpose. On opening such a shell, it is seen that the two valves 

 are lined by a membrane, in which the animal is enclosed, like a 

 book between the boards of its cover. This membrane is divided 

 into two halves along a considerable part of the edge of the 

 valves ; but is united near the large end. In some Conchifera, 

 as will be presently noticed, the two valves of the mantle are 

 separated along their whole extent ; whilst in others they are 

 completely closed, with the exception of the two orifices for the 

 ingress and egress of water ; which are sometimes drawn out 

 into long tubes. In the Mactra, the water enters through one 

 of the short respiratory tubes, and passes out by the other ; but 

 the water thus introduced is principally for the supply of the 

 gills, the mouth, or entrance to the stomach, being placed at the 

 other end of the shell, where the mantle is quite open ; and being 

 thus able to take in food from the surrounding water, which comes 

 into free contact with it. The gills in all Lamellibranchiata 

 consist of four riband-like fringes, fixed to the mantle along the 

 edge of the shell most distant from the hinge. Near the middle 

 of the shell is seen the stomach, with the short tube leading to it, 

 the orifice of which (or mouth) is furnished with four tentacula 

 or feelers. To the left of this is seen the long and complicated 

 intestinal tube, with the liver lying in separate masses amongst 

 its folds. Below this is seen the ovarium, in which the eggs are 



