CLASSIFICATION. 93 
CLASS II. 
GASTEROPODA. 
Animal. Body free, without limbs to aid progres- 
sion ; but with an abdominal expansion or foot 
suitable for crawling, and in a small number for 
swimming ; head distinct, bearing commonly one 
or more pairs of tentacles, and almost ahvays eyes 
variously situated upon or near the tentacles ; res- 
piratory organs branchial, very variable in their 
form and position. 
Shell. External, internal, or wanting, almost al- 
ways a single piece, conical or spiral ivhen exter- 
nal, and more or less rudimentary when internal, 
sometimes operculated. Inhabiting the sea, fresh 
water, and land. 
The subdivisions of this class, or the Orders, have, 
in nearly all the methods, been founded upon differ- 
ences in the structure of the respiratory apparatus, and 
the genera are grouped into them, according to their 
supposed relations in this particular, and vrithout ref- 
erence to their other characters. But, the arrange- 
ment of the genera is not the same in any two of 
them, and the same terms being used by different au- 
vol. I. 25 
