HOW ANIMALS BREATHE 



321 



FIG. 276! Spiracle of an Insect, x 75. 



to streams of water. The gills are always covered. In 



bony fishes they are attached to the hinder side of bony 



arches, all covered by 



a flap of the skin, 



supported by bones 



(the gill cover, or 



operculum\ and the 



water escapes from 



the opening left at 



its hinder edge. In 



sharks, the gills are 



placed in pouches 



which open separately 



(Figs. 122,360). The 



act of " breathing 



water" resembles 



swallowing, only the water passes over the surface of 



the gills instead of entering the gullet. 



(2) Air Breathers have trachea, or lungs. The former 



consist of two principal tubes, which pass from one end 



of the body to the other, opening on the surface by 

 apertures, called spiracles, resembling a 

 row of buttonholes along the sides of 

 the thorax and abdomen, and ramifying 

 through the smallest and most delicate 

 organs, so that the air rriay follow the 

 blood wherever it circulates.. To keep 

 the pipes ever open, and at the same 

 . time leave them flexible, they are pro- 



FG. 277. Tracheal 



inside with an elastic spiral thread, 

 a droplight. 



Respiration is performed by the move- 

 ments of the abdomen, as may be seen in the bee when 

 at rest. This " air-pipe system," as it may be termed, 

 is best developed in insects. 



Tube of an Insect, 

 highly magnified, 



showing elastic like the rubber tube of 



spiral thread. 



DODGE'S GEN. ZOOL. 21 



