THE CLASS OF INSECTS. 3 



ramify throughout the whole interior of the animal, and con- 

 nect with breathing pores (stigmata) in the sides of the body. 

 They do not breathe through the mouth as do the higher ani- 

 mals. The tracheae and blood-vessels follow closely the same 



Fig. 4. 



course, so that the aeration of the blood goes on, apparently, 

 over the whole interior of the body, not being confined to a 

 single region, as in the lungs of the vertebrate animals. 



Thus it is by observing the general form of the body-walls, 

 and the situation of the different anatomical systems, both in 

 relation to themselves and the walls of the body, or crust, 

 which surrounds and protects the more delicate organs within, 

 that we are able to find satisfactory characters for isolating, in 

 our definitions, the articulates from all other animals. 



We shall perceive more clearly the differences between the 

 three classes of Articulates, or jointed animals, namely, 

 the WORMS, CRUSTACEANS, and INSECTS, by examining 



often as lungs. The nervous cord (a) rests on the floor of the cylinder, sending a 

 filament into the oar-like feet (/), and also around the intestine or stomach (6), to a 

 supplementary cord (d), which is situated just over the intestine, and under the 

 heart or dorsal vessel (c). The circle c and e is a diagram of the circulatory sys- 

 tem ; c is the dorsal vessel, or heart, from the side of which, in each ring, a small 

 vessel is sent downwards and around to e, the ventral vessel. Original. 



FIG. 4. An ideal section of a Bee. Here the crust is dense and thick, to which 

 strong muscles are attached. On the upper side of the ring the wings grow out, 

 while the legs are inserted near the under side. The tracheae (d) enter through the 

 stif/ma, or breathing pore, situated just under the wing, and their branches sub- 

 divide and are distributed to the wings, with their five principal veins as indicated 



