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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 subcesophageal group of ganglia. 



SINCE Cuvier several different groups of animals, which were placed 

 amongst the worms by Linnaeus, have 

 been included in the Mollusca. Of late 

 years, however, the anatomy and de- 

 velopment of these 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 Gasteropod 

 some time been the case. The bivalved fJ^'^z^Sum^ T tentacles 1 - o' 

 Brachiopoda, 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, " M6moires pour servir a 1'histoire et & 1'anatomie des Mol- 

 lusques." Paris, 1817. 



R. 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. 



