THE LAMELLIBRANCHIATA. 



35 



extends between the foot and the mantle lobes in front, and 

 between the mantle lobes posteriorly (Fig. 14). The branchial 

 organs may consist of distinct filaments, or of plates composed 

 of tubular rods supporting a network of blood-vessels, and 

 covered with cilia, by the action of which they are constantly 

 bathed by currents of water. 



Fig. 14. Anodon, vertical and transverse section of the body through the heart ; /, ven- 

 tricle ; g, auricles ; c, rectum ; p, pericardium ; h, inner, ', outer gill ; o'q, organ 

 of Bojanus ; B, foot ; A A, mantle lobes. 



The nervous system presents a no less distinct advance than 

 the other organs. All Lamellibranchs possess at least three 

 pairs of principal ganglia a cerebral pair at the sides of the 

 mouth, a pedal pair in the foot, and a third pair on the under 

 surface of the posterior adductor muscle, which are commonly 

 called " branchial," but which, as they supply not only branchial, 

 but visceral and pallia! filaments, may more properly be termed 

 " parieto-splanchnic." Three sets of comrnissural filaments con- 

 nect the cerebral ganglia with one another, with the pedal, and 

 with the parieto-splanchnic ganglia. The inter-cerebral com- 

 missures surround the mouth, and the other two pairs of cords 

 extend respectively from the cerebral to the pedal, and from 

 the cerebral to the parieto-splanchnic ganglia. 



Finally, there is always, in these animals, an external shell, 



D 2 



