INTRODUCTION. 



Of the great or primary assemblages into which all ani- 

 mals may be grouped, the Mollusca constitute one. They 

 rank inferior to the Vertebrata, partially inferior to the 

 Articulata, and mainly superior to the Radiata and 

 Amorphozoa. From the vertebrate animals they are con- 

 spicuously distinguished by their limb-less bodies, and the 

 absence of a jointed internal skeleton ; from the articulate 

 animals, by their inarticulated exoskeleton, and bodies not 

 divided into segments ; from the radiate animals, zoo- 

 phytes and sponges, by their more complex organization 

 and subsymmetrical non-radiated forms. Between them 

 and certain animals of all the other groups, there are, how- 

 ever, striking resemblances of analogy. The mollusks are 

 all provided with a well-defined nervous system, the dis- 

 tinctive feature of which is that of being heterogangliate. 

 They possess distinct organs of digestion, respiration, and 

 circulation. The rapid anamorphosis of the entire group, 

 shown by the progression from a low type of organ- 

 ization, such as is seen in the Bryozoa^ to one compara- 

 tively very high, as exhibited by the Cephalopoda, neces- 

 sarily implies varied manifestations of internal structure 

 and external form. Hence the subdivisions of the Mollusca 

 display remarkable contrasts in their diagnostic characters, 

 and the aspect of a member of one of the lower divisions 



