CHAPTER I. 



MOLLUSCA.* 



Bilaterally symmetrical unsegmented animals, without a locomotory 

 skeleton ; with a ventral foot and usually a calcareous univalve or 

 bivalve shell ; with brain (supracesophageal ganglia), circumoesophageal 

 ring, and subo&sophageal group of ganglia. 



SINCE Cuvier several different groups of animals, which were placed 

 amongst the worms by Linnaeus, have 

 been included in the Mollusc^. Of late 

 years, however, the anatomy and de- 

 velopment of \hese forms have been 

 more closely examined, and it seems 

 fairly certain that some of them are 

 allied to the Worms. In any case, the 

 group Mollusca must be looked upon /' 



as of more limited extent than has for FIG. 492. Older larva of a Oasteropod 



. i ,1 rm i i -i (after Gregenbaur). S, Shell ; P, 



some time been the case. The bivalved J oot . Frf * velum; r , tentacles; P , 



Erachiopoda, which in structure and opercuium for the closure of the 



development stand in closer relationship 



to the Bryozoa,, may be removed from the Mollusca and united with 



the latter under the head Molluscoidea. The Tunicata also must 



be constituted an independent group between the Mollusca and the 



Vertebrata. 



* G. Cuvier, " Memoires pour servir a 1'histoire et & 1'anatomie des Mol- 

 lusques." Paris, 1817. 



K. Leuckart, " Ueber die Morphologic und die Verwandschaftsverhaltnisse 

 der wirbellosen Thiere." Braunschweig, 1848. 



Huxley, " On the Morphology of the Cephalous Mollusca, as illustrated 

 by the Anatomy of certain Heteropoda and Pteropoda, etc." Phil. Trans., 

 1853. 



