78 THE NERVOUS SYSTEM OF MOLLUSKS. 



by means of gills. Bat being all of tbem, as Prof. Owen 

 says, " endowed with power to attain, subdue, and devour 

 organic matter, dead and living," we find their Nervous 

 System not only better developed, more complex and con- 

 centrated, but also in relation with more highly evolved 

 organs of special sense and exploration. This system 

 offers considerable variations in general arrangement, and 

 as regards the relative positions of its ganglia, though 

 these modifications are, to a great extent, referable to 

 differences in the outward configuration of the body. 



The wide differences in external form which are to be 

 met with among Gasteropods may be well illustrated by 

 comparing the Limpet or the Chiton with the Snail. 

 Here differences in habit are also marked, so that we 

 almost necessarily meet with very notable variations in the 

 disposition of the principal parts of the nervous system. 



In the Limpet two small cerebral ganglia (fig. 24 a) 

 exist, which are widely separated from one another, and 

 lie at the side of the oesophagus. Each 

 receives a rather large nerve from one of 

 the tentacles, and a smaller optic nerve. 

 A commissure above the oesophagus con- 

 nects these cerebral ganglia with one 

 ^^n'f^f^i\. another, while each of them is also in 

 ^U^ \\\ relation by means of two descending 

 Fiu. -21. — Nervous commissurcs with a series of four con- 

 system of Common ^ -^ ganglia forming a transversely 



Limpet. (Todd after fc- o o J 



Garner.) a, Cerebral arranged row beucath the oesophagus. 

 !n7t peda/gan^r! Of thcse thc two median ganglia (h) 

 D, pharyngeal and K corrcspoud witli the pcdal, wliilc the two 



labial ganglia. (/ and ''>..,. t .,■^ ,-, ■, 



Commissures ; p, tcnta- cxtcmal (c) corrcspoud With tiic orau- 

 cuiar nerve; i, optic ^^j^'g^j ganglia, though they are here 



separated from one another by an un- 

 usually wide interval. 



