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 GASTEROPODS ; 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 



* d / 



Fig. 157. ANATOMY OF SNAIL. 



/", muscular disc or foot; t, tentacula; d, diaphragm separating the respiratory 

 cavity p from other organs, but here turned back; s, stomach; o, ovary; or, 

 arterial trunk supplying the system; i, r, intestine; I, liver; h, heart ; ap, vascular 

 trunk spreading over the pulmonary cavity p ; cv, canal for excreting the viscid 

 mucus secreted by the gland v. 



the external air, and having the blood minutely distributed by 

 vessels upon its walls. In the MYRIAPODA 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 INSECTS, the same general arrange- 

 ment is modified in the most remarkable manner. 

 The spiracles do not open into distinct air-sacs, 

 but into canals, which lead to two large tracheae 

 which run along the sides of the body, and are 

 connected by several tubes that pass across it 

 one usually for each segment. From these 

 tracheae others branch off, which again subdivide into more 



Fig. 158. 



AIB.-TUBE OP IN- 

 SECT. 



