Seventh Section. 



Mollusca. 



General Review of the Group. 



§ 247. 



The general characters of the body, and of its various systems of 

 organs, distinctly define the phylum of the Mollusca. Owing to the 

 absence of any distinctly-marked external metamerism, the body 

 appears to be more compact than in the Arthropoda or Annulata 

 among Vermes ; indications of metamerism may, however, be made 

 out in various organs. The supracesophageal position of the 

 central nervous system, and its connection with lower-lying ganglia, 

 or with commissures surrounding the pharynx, when taken in con- 

 nection with the position of the heart, which is always dorsal, are the 

 definitely typical characters of this division ; to which we may add 

 that in most forms shells are developed from the dorsal surface. 



The complete disappearance of their primitive metamerism, and 

 the gaps that there are between the classes here united together, are 

 completely explained by the early appearance (palasontologically 

 speakiug) of most of the classes of the Mollusca ; while the forms 

 still living are seen to be an exceedingly small part of the phylum, 

 which was rich in forms, but which has been continued on in a 

 relatively small number of divisions. As yet we know very little of 

 the phylogeny of the Mollusca, but their metameric arrangements, 

 as indicated by their internal organisation, point to then affinity to 

 segmented organisms, the nearest allies of which were some of the 

 Vermes. 



Although we can classify the various orders as higher and lower, 

 yet all the systems of organs have not been developed to the same 

 extent, so that we are able to find distinct proofs of the affinity 

 between every single division and low T er forms. 



