92 INTRODUCTION. 
indicates correctly in all its details, tlie 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 Blainville : 
Mollusca. Animals icith two equal, symmetrical 
halves; the body with its appendages soft, not articu- 
lated, enveloped in a skin 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 Hood; heart essentially 
aortic and above the intestinal canal. Respiration 
aquatic, or aerial. Nervous system composed of a cere- 
briform 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. Acepuala. In which a distinct head is wanting. 
The second class, which includes the animals herein 
described, is characterized by M. Rang, as follows : 
