24 Animal Life and Intelligence. 



acid gas. In the frog there is no diaphragm, and there 

 are no ribs. The lungs are hollow sacs with honey-combed 

 sides, and they are inflated from the mouth, which is 

 used as a force-pump for this purpose. In the fish there 

 are no lungs, respiration being effected by means of gills. 

 In these organs the blood is separated from the water 

 which passes over them (being gulped in by the mouth 

 and forced out between the gill- covers) by only a thin 

 organic film, so that it can take up the oxygen dissolved in 

 the water, and give up to the water the carbonic acid it 

 contains. In fishes, too, we have only one receiver and 

 one force-pump, the blood passing through the gills on its 

 way to the various parts of the body. In the lobster, again, 

 there are gills, but the mechanism by which the water is 

 drawn over them is quite different, and the blood passes 

 through them ,on its way to the heart, after passing 

 through the various organs of the body, not on its way 

 from the heart, as in vertebrate fishes. The blood, too, 

 has no red blood-discs. In the air-breathing insects the 

 mechanism is, again, altogether different. The air, which 

 obtains access to the body by spiracles in the sides (see 

 Fig. 1, p. 3), is distributed by delicate and beautiful tubes 

 to all parts of the organs ; so that the oxygen is supplied 

 to the tissues directly, and not through the intervention of 

 a blood-stream. In the earthworm, on the other hand, 

 there is a distributing blood-stream, but there is no 

 mechanism for introducing the air within the body ; while 

 in some of the lowliest forms of life there is neither any 

 introduction of air within the body nor any distribution 

 by means of a circulating fluid. Beginning, therefore, 

 with the surface of the body simply absorbent of oxygen, 

 we have the concentration of the absorbent parts in special 

 regions, and an increase in the absorbent surface, either 

 (1) by the pushing out of processes into the surrounding 

 medium, as in gills; or (2) by the formation of internal 

 cavities, tubes, or branching passages, as in lungs and the 

 tracheal air-system of insects. 



What, then, is the essential nature of the respiratory 



