272 ELEMENTS OF COMPARATIVE ZOOLOGY. 



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. 



The nervous system consists of at least three pairs of 

 ganglia and the cords or commissures connecting 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 in- 

 testine; 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 correspondingly 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 darkness, and 

 from these all degrees of development may be found to the 

 extreme in the squid, where these organs are scarcely infe- 

 rior to those of vertebrates in structure. 



For kidneys the molluscs have one or two organs consist- 

 ing of convoluted tubes opening at their inner end into the 

 pericardium and communicating with the exterior at the 

 other. 



In some the sexes are separate ; in others, like our land 



