82 



FIRST LESSONS IN ZOOLOGY 



the g-ills and pumps the blood directly into and through 

 them into vessels that carry it all over the body. The 



fish keeps a current of 

 water passing over its 

 gills. First it opens 

 the mouth, spreading 

 the gill covers, when 

 the water rushes in, 

 after which it closes 

 the mouth drawing the 

 cover bent covers together so that 



the water is forced back 



Fig. 45.— Head of tmutwith gi 

 forward t(j show gills- (From specimen.) 



over the gills. This operation is constantly repeated. 

 If the mouth of an active fish 

 like a sunfish is fastened open it 

 will die, since it can no longer 

 breathe. It must be added 

 that the fins of most fishes no 

 doubt aid somewhat in respira- 

 tion since they are well supplied 

 with blood and the skin on 

 them is very thin. 



Insects are mostly air-breath- 

 ers. Man)', the bee and dragon- 

 flyfor example, show very great 

 activity, demanding much oxy- 

 gen. They have an elaborate 



r 



system of tubes, called trachea; tr 

 (figs. 46, 47, and 48), which 

 penetrate every part of the 

 body, reaching in some cases 

 every muscle cell. These open 

 to the air by means of pairs 

 of small holes, called spiracles, 

 in most of the body segments. 



x'sp 



Fig. 46. — Diagram showing tra- 

 che.il s)-stein of a beetle; sp, 

 sj^iracies; /r, traeheiv. (After 

 Kollie.) 



Air is made to come in 



