ft^DING 



^NO 



LTHOUGH it is not a part of my 

 purpose to enter upon the anatomy 

 and physiology of the MoUusca 

 beyond what is necessary for an 

 understanding of the differences in 

 form and habit of the various 

 groups, there are certain points which are so char- 

 acteristic of the class, or which distinguish the 

 several orders, that some elementary reference to 

 them is essential. But the reader whose interest 

 at present extends only to external forms and 

 coverings may, an it please him, for the present 

 skip this chapter and return to it later. 



Thoupfh, as we have seen, the bivalves have 

 probably retrograded so far from the prevailing 

 type as to have lost their heads, these presumed 

 degenerates have retained a mouth and a digestive 

 system as being necessary to their continued 

 existence. But though the mouth remains in these 

 bivalves, it is little more than an opening to the 



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