x PHYLUM ARTHROPODA 241 



of a poison gland. In the millipedes each segment behind 

 the fourth or fifth bears two pairs of legs, the four or five 

 most anterior having only one pair each, except one segment 

 which is footless. In most of the millipedes and their allies 

 the appendages of the seventh segment are modified in the 

 male to form copulatory organs. 



The enteric canal is straight and is much simpler in 

 character than that of the Insecta. The heart is in the form 

 of a long tube, consisting of as many chambers as there are 

 segments in the body. The breathing organs are air-tubes 

 or tracheae, resembling those of larval insects, such as cater- 

 pillars. 



4. THE INSECTA 



The class Insecta, comprising the cockroaches, grass- 

 hoppers, dragon-flies, beetles, butterflies, house-flies, and 

 bees, with their many allies, though it is a very extensive one, 

 including as it does a larger number of species than any 

 of the other classes of the Arthropoda, is yet characterised 

 by a remarkable degree of uniformity, no such extremes of 

 modification occurring as are observable among the Crustacea. 

 The body of an insect, like that of a crustacean, is segmented, 

 and bears a series of pairs of jointed appendages. The 

 surface is covered with a chitinous cuticle, forming an exo- 

 skeleton, which is sometimes comparatively thin, sometimes 

 thick and hard. Like the body of the crustacean, that of 

 the insect is divisible into certain regions. In the Insecta 

 these regions are quite constant in their disposition, and are 

 always three in number, head\i\ front, thorax in the middle, 

 and abdomen behind. The head is found, when its develop- 

 ment is traced, to be formed by the union with the head-lobe 

 of the embryo of some five segments, but in the adult no 

 trace of segmentation is distinguishable. The thorax always 



