416 



GASTEROPODA. 



In Pterotrachea Fi ff- 193. 



the same dispersion 

 of the central gan- 

 glia of the nervous 

 system is equally 

 evident. The brain 

 and nervous collar 

 around the oesopha- 

 gus occupy their 

 usual situation, and 

 give nerves to the 

 tentacles, eyes, and 

 parts around the 

 mouth ; while four 

 smaller ganglia 

 (Jig. 188, f) are 

 placed in the imme- 

 diate vicinity of the 

 foot, to which, and 

 to the neighbour- 

 ing viscera, they 

 distribute their 

 branches. 



But in the most 

 elevated Gastero- 

 pods the ganglia 

 assume greater con- 

 centration, and the brain exhibits much larger dimensions as 

 compared with the size of the body. Thus in the snail (Jig. 

 184) we find only two great nervous masses : the brain (7), a large 

 ganglion placed above the oesophagus, and supplying the nerves 

 connected with sensation ; and an equally large suboesophageal 

 mass (m) 9 whence proceed nerves to all the viscera and locomotive 

 organs. Here, therefore, we have another example of the great 

 law that we have already so often illustrated the diminution in 

 number, and the increase in size, of the nervous centres as we rise 

 from lower to more exalted types of animal organization. 



The tentacula (Jig. 193,/,/) in the marine GASTEROPODA are 

 generally not retractile, and the eyes are frequently situated at the 

 outer side of the base of each tentacle, instead of at their apex, as 

 in the figure referred to ; but, with these exceptions, we can add 



