GENERAL STRUCTURE OF LAMELLIBRANCHIATA. 399 



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

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

 terior 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. 685, c). Upon this character it has 

 been proposed to found the primary division of the Class into Or- 

 ders ; but the classification thus formed is not a natural one, inas- 

 much as it brings together kinds which have little resemblance, 

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

 685 are shown the several parts of a Bivalve Shell, with the 

 explanation of the names by which they are Described. 



1017. In order to describe the general structure of the Lamelli- 

 branchiata, it will be advantageous to select some particular 

 illustration ; and the Mactra (Fig. 686) 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 Lamellibran- 

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

 along the edge of the shell most distant from the hinge. The 

 gills, as well as the interior of the siphons or respiratory tube and 

 the membrane of the mantle, are covered with innumerable 

 minute vibratile cilia, and it is by the agency of these organs that 

 the currents are established in the water, by which the constant 

 change of that element, necessary for the performance of the 



