106 SYSTEMATIC ZOOLOGY. 



The teeth on the ribbon vary in number and shape in 

 different species. In some there are but three in a trans- 

 verse row, while in others there may be over one hundred. 



In the ideal mollusc the alimentary canal goes straight 

 through the body from mouth to vent. In nature it 

 usually has some convolutions, increasing the amount of 

 digestive surface. In the cephalopods and in most gas- 

 teropods it becomes bent on itself, so that the vent is far 

 in front, either upon the right side or even in the median 

 line. In the gasteropods, when it is median, it is close to 

 and dorsal to the mouth. In the cephalopods it is ventral. 

 A large liver is always present. 



The nervous system (fig. 34, right) consists of at least 

 three pairs of ganglia and the cords or commissures con- 

 necting them, as well as the nerves going to the various 

 parts. These ganglia are the cerebral, above the mouth; 

 the pedal, primarily in the foot; and the visceral, farther 

 back in the body. Both pedal and visceral ganglia are 

 below the intestine; the pedal supplying the foot, the 

 visceral the body and the mantle. To these three pairs 

 others are frequently added. Sometimes the ganglia are 

 widely separated, when the commissures are correspond- 

 ingly lengthened; or they may be brought close together, 

 with shortened connecting cords. 



Some molluscs lack organs of special sense; others have 

 eyes and ears. The ears are little sacs, usually near the 

 pedal ganglion, but the eyes may have various positions. 

 They may be on the sides of the head (squid), or on the 

 sides or tips of tentacles arising from the head (snails), or 

 scattered over the back (some slugs and chitons), or on 

 the edges of the mantle (scallops), or on the end of the 

 siphon (some clams). In some they are merely spots 

 which have the power to distinguish between light and 



