74 INSECTS AND 
or dorsal vessel, and below, upon the under side, rests 
the nervous system. The breathing apparatus, or “lungs,” 
in Worms consists of simple filaments, placed on the front 
of the head; or of gill-like processes, as in the Crustacea, 
which form simple expansions of the legs; or, as in 
the Insects, of delicate tubes (trachea), woe ramify 
throughout the whole interior of the animal, and connect 
with breathing pores (stigmata) in the sides of the body. 
They do not breathe through the mouth as do the higher 
animals. The traches and blood-vessels follow closely 
the same course, so that the aération 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 verte- 
brate 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 charac- i 
ters for isolating, in our os the articulates from 
all other animals. 
ts ed We shall perceive more clearly the differences 
