6 DEFINITION OF THE TERM INSECT. 
marck. Thus the first of Aristotle’s great divisions he 
increased by the addition of anew and very distinct class, 
the Amphibia, by which some ground was gained in the 
science; but as much was lost by his compressing the 
four classes of which the last consisted into two, by which 
the natural classes of Cephalopoda and Crustacea merged 
under Insecta and Vermes. Linné was not aware of the 
extraordinary fact, that the Cephalopoda have three 
hearts; and that though the Crustacea and Arachnida 
have a circulation, Jnsects have none, or he would never 
have taken this retrograde step. 
Indeed Linné’s definition of an Insect is, in many 
most material points, inapplicable, not only to the Crus- 
tacea, but to many other animals included under that 
denomination. This will appear evident from a very 
slight examination. Thus it runs: “ Polypod animal- 
cula, breathing by lateral spiracles, armed every ‘where 
with an osseous skin, whose head is furnished with mov- 
able sensitive antenne*.” Now of this definition only 
the first member can be applied to the whole class which 
it is meant to designate; for the entire genus Cancer L., 
which, with some others, forms the class Crustacea of the 
moderns, does not respire by spiracles at all, but by gells ; 
and the same in some degree may be said of spiders, 
scorpions, &c. With the last member of the definition 
Linné himself must have been aware that a large number 
of what he conceived to be insects were at variance, as 
mites, spiders, and many other of his apterous tribes : 
* Animalcula polypoda, spiraculis lateralibus respirantia, cute 
ossea cataphracta ; antennis mobilibus sensoriis instruuntur. Sie. 
Nat. ed. 12. 1, 533. 
