92 INTRODUCTION. 



indicates correctly in all its details, the present state 

 of this science, and hence, no one of them is followed 

 in full, in this work ; but, such of the views of dif- 

 ferent authors as are acceptable to us, are with some 

 alteration adopted here. The Mollusks are thus de- 

 fined by M. de BlamvOle : 



MoLLUSCA. Animals with two> equal, symmetrieal 

 halves; the body with its appendages soft, not articu- 

 lated, enveloped in a shin or muscular membrane 

 (mantle) of various form, upon or within which is de- 

 veloped a calcareous shell of one or more pieces. Cir- 

 culation complete, with white blood; heart essentially 

 aortic and above the intestinal canal. Respiration 

 aquatic, or aerial. Nervous system composed of a cere- 

 Iriform ganglion above the oesophagus, communicat- 

 ing with the ganglia of the different functions, those 

 of locomotion being lateral. 



They were divided by M. Cuvier into three Classes. 



Class I. Cephalopoda. In which the locomotive organs are 



attached to the head. 

 Class II. Gasteropoda. In which locomotion is performed 



by an expansion of the ventral disk forming a 



sort of foot. 

 Class III. AcEPHALA. In which a distinct head is wanting. 



The second class, which includes the animals herein 

 described, is characterized by M. Rang, as follows ; 



