274 ATMOSPHERIC RESPIRATION. 
320, In animals whose blood is made to act directly upon 
the air, we usually find a provision of some kind for intro- 
ducing the air into the interior of the body. The simplest 
arrangement is that which we meet-with in the Snail and other 
terrestrial GasTeRoPops ; and it consists merely of a cavity 
(p, fig. 157), resembling that in which the gills are disposed in 
the aquatic Mollusca, but having a free communication with 
h 
| 
amen 
Fig. 157.—ANAToMY OF SNAIL. 
f, muscular disc or foot; ¢, tentacula; d, diaphragm separating the respiratory 
cavity p from other organs, but here turned back; s, stomach; o, ovary; ar, 
arterial trunk supplying the system; 4, r, intestine; J, liver; 4, heart; ap, vascular 
trunk spreading over the pulmonary cavity p; ev, canal for excreting the viscid 
mucus secreted by the gland o, 
the external air, and having the blood minutely distributed by 
vessels upon its walls.—In the Myrrapopa or Centipede tribe, 
in conformity with the general plan of Articulated structure 
(§ 93), we find a repetition of similar cavities along the body, 
one pair usually existing in each segment ; 
and these open externally by small apertures, 
which are termed spiracles. 
321. In Inszots, the same general. arrange- 
mentis modified in themostremarkablemanner. 
The spiracles do not open into distinct air-saes, 
but into canals, which lead to two large trachex 
Fig. 188 which run along the sides of the body, and are 
Armruse or Ix. connected by several tubes that pass across it— 
sxct. one usually for each segment. From these 
traches others branch off, which again subdivide into more 
