33 



luscan characteristics have been modified as the result of a 

 more or less stationary mode of life. The head region, 

 including the prostomium, the cephalic tentacles, and the 

 eyes, has become more and more rudimentary so as to be 

 finally suppressed altogether. The primitive bilateral sym- 

 metry is retained, but the body is much compressed from 

 side to side, the result being that the foot is either nearly 

 aborted or is reduced to a narrow projection. In conse- 

 quence of the flattening, the mantle is also changed and 

 takes the form of two large laterally placed lobes, on the 

 outer surface of each of which a valve or plate of the shell is 

 formed. The ctenidia have become enormously enlarged to 

 form two great gill-lamellae on each side of the body. 

 Besides being respiratory in function, they also, by their 

 extended ciliated surface, cause currents of water which 

 bring food particles to the mouth — an important matter to 

 an animal with very limited powers of movement. The 

 cerebral, pleural, and visceral ganglia of each side of the 

 body have coalesced to form a single mass, the so-called 

 cerebral ganglion, placed alongside the mouth. The pedal 

 ganglia are normal, and the osphradial are very large 

 and are usually known as parieto-splanchnic. Two large 

 muscles run transversely across the body, one in front of and 

 above the mouth, and the other below the anus. They serve 

 to approximate the valves of the shell, and are the anterior 

 and posterior adductor muscles. 



In the primitive Lamellibranchs, these muscles were 

 probably of much the same size, and we find the most direct 

 descendants of such forms in the Isomya of the present 

 day, and especially in the Integropalliata, such as Area, 

 Nucula, and Trigonia, which may be regarded as being 

 more in the direct line of development than any of the 

 other Lamellibranchiata. From this stock two divergent 

 lines have sprung. On the one hand, the Sinupalliata have 



