GENERAL STRUCTURE OF LAMELLIBRANCHIATA. *393 



functions, is as follows : The branchial laminae consist of 

 numerous delicate tubes running parallel to each other from the 

 base towards the free edge. The walls of these tubes are formed 

 by a network of delicate blood-vessels, through the meshes of 

 which the water bathing the outer surface of the gills is, as it 

 were, filtered into the little tubes permeating their interior, 

 through which it passes into the excurrcnt siphon. Any particles 

 of nutritive matter which may be contained in the water, are 

 arrested by the fine, sieve-like structure of the gills, in the little 

 furrows of the surface of which they are collected into small 

 thread-like masses. These are transferred by the action of the 

 cilia to the grooved edge of the gill, along which they are carried 

 in the same way until they reach the mouth. 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 

 formed; this occupies a large part cf the cavity of the shell 

 during the breeding season. Close to this is the posterior ad- 

 ductor muscle ; by which, with the aid of the anterior muscle 

 situated near the mouth, the valves can be drawn together with 

 considerable force. The intestinal tube is seen to terminate near 

 the opening at the posterior extremity of the shell, in one of the 

 respiratory tubes, which discharges its contents, and serves for 

 the exit of the respiratory current. 



1018. The foregoing description will apply, with slight varia- 

 tions, to the structure of almost all Lamellibranchiata ; but we 

 have now to notice two organs, which are absent in some, and in 

 others more largely developed than in the present instance. At 

 the left side of the figure, projecting beyond the edge of the 

 mantle, is seen the foot ; a fleshy muscular organ, somewhat 

 resembling the tongue of higher animals, and not containing 

 any hard support, or protected by any envelope. This foot, 

 which is the only special locomotive organ possessed by the 

 Mollusca of this class, serves a great variety of purposes ; some- 

 times enabling the animal to leap with considerable agility along 



