CIRCULATION AND RESPIRATION IN ANIMALS 169 



ment: an intake of free oxygen for combustion, and an outgo 

 of the waste products, chiefly carbon dioxide. In the unicel- 

 lular organisms, such as Sphaerella and Paramecium, and in 

 simple multicellular animals like Hydra, this appears to be a 

 relatively simple process since an elaborate mechanism is not 

 necessary to facilitate the interchange. But with the estab- 

 lishment of a highly differentiated multicellular body, fewer 

 and fewer cells are in direct contact with the aerating medium 

 and so various provisions are necessary to transfer the gases 

 to and from the outer world and the individual cells them- 

 selves. In all forms the skin functions to some extent; in 

 the Earthworm, in fact, it acts as the chief respiratory 

 membrane since a profuse supply of blood vessels to the 

 moist surface of the body effects a sufficiently rapid gaseous 

 interchange for the relatively inactive life of the organism. 

 The Crayfish meets the problem of respiration by finger-form 

 out-pocketings of the body wall, the gills: a method of bath- 

 ing a large area of the respiratory membrane in the respiratory 

 medium, the surrounding water. The Insects, on the other 

 hand, instead of bringing the blood to the surface, develop a 

 network of tubes which ramify throughout the body and 

 conduct the air directly to the various tissues. Among the 

 lower Vertebrates, as has been indicated, the anterior end of 

 the digestive tract functions as a common food and respira- 

 tory passage. In Fishes, the respiratory water current which 

 enters the mouth makes its exit by way of the gill pouches 

 and gill slits; the lining of the pouches outpocketings of 

 the lining of the alimentary canal functioning as the res- 

 piratory membrane. (Fig. 94.) 



Among the air-breathing Vertebrates tnere are the added 

 problems of protecting and keeping moist the greatly in- 

 creased respiratory surface which their active metabolism 

 demands. Accordingly the gill pouches are replaced by a 



