RESPIRATION 



211 



477- The crayfish bears under a fold of the carapace, on either 

 side, a large number of brush-like gills, composed essentially of 

 numerous slender thin-walled filaments, through which the 

 blood constantly circulates. There is also a special structure 

 in the form of a curved paddle or spoon, which by its motion 

 keeps the water constantly moving through the gill chamber. 

 This highly efficient set of organs evidently 

 makes good the deficiency in absorbing power 

 of the body surface resulting from the imper- 

 vious cuticular integument. 



478. In Insects, a unique method of aerating 

 the body has developed. The air is carried to 

 all parts of the body by an intricate system of 

 slender tubes, tracheae, which open on the sur- 

 face through small pores in the integument, the 

 stigmata. The air is forced into and out of 

 these tubes by a telescoping action of the rings 

 of the abdomen. 



479. In Fishes, the gills are not unlike those 

 of the crayfish, but the water current is produced 

 in a different way. The water is first taken into 

 the pharynx through the mouth, and from the 

 pharynx it passes through a series of slits be- 

 tween the arches which bear the gills. A pair of delicate 

 membranes at the mouth serve as valves and cause a flow of 

 water, always in the same direction, to result from merely 

 opening and closing the mouth. 



480. In a few fishes and in the adult stage of all other W'rte- 

 brates, a pair of air sacks or lungs take the place of the gills of 

 the fish. In the lower forms the lungs are comparatively 

 simple, the inner surface of the air sacks being, at most, some- 

 what folded. In the birds and mammals, however, they 

 become exceedingly complex, through the folding of the walls to 

 increase the absorbing surface. The lung first appears as a 



lu.. 1 1 7. — Dia- 

 gram of a fcalhcr- 

 likc gill- This 

 type is found in 

 the crayfish. 



