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 inferioi- 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 



