6 DEFINITION OF THE TERM INSECT. 



marck. Thus the first of Aristotle's great divisions he 

 increased by the addition of a new 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. Linne was not aware of the 

 extraordinary fact, that the Cephalopoda have three 

 hearts; and that though the Crustacea and Arachnida 

 have a circulation, Insects have none, or he would never 

 have taken this retrograde step. 



Indeed Linne'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 antenna*" 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 gills ; 

 and the same in some degree may be said of spiders, 

 scorpions, &c. With the last member of the definition 

 Linne 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 : 

 though from some very recent observations of M. La- 



a Animalcula polypoda, spiracidis lateralibus respirantia, cute 

 ossea eataphracta; antennis mobilibus sensoriis instruuntur. Syst. 

 Nat, ed. 12. i. 533. 



