CHAPTER XII 

 RESPIRATION 



The living body is the theatre of many chemical and physical oper- 

 ations in line with those of the inorganic domain. — Thomson. 



As we have seen, the essential factor of respiration is an inter- 

 change of gases between protoplasm and the environment: an 

 intake of free oxygen for combustion, and an outgo of the waste 

 products, chiefly carbon dioxide and water. In the unicellular or- 

 ganisms, such as Protococcus and Amoeba, and in simple multi- 

 cellular animals like Hydra, this appears to be a relatively simple 

 process since an elaborate mechanism is not necessary to facilitate 

 the interchange. Rut with the establishment of a highly differen- 

 tiated multicellular body, fewer and fewer cells are in direct con- 

 tact with the aerating medium and so various provisions are 

 necessary to transfer the gases to and from the outer world and 

 the individual cells themselves. 



In all forms the skin functions to some extent; in the Earth- 

 worm, 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 outpocketings of the body wall, the 

 gills: a method of bathing a large area of the respiratory mem- 

 brane in the respiratory medium, the surrounding water. Insects, 

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

 network of tubes, or tracheae, which ramify throughout the body 

 tissues and conduct air directly to the cells. (Fig. 116.) 



Among the lower Vertebrates, as has been indicated, the an- 

 terior end of the digestive tract functions as a common food and 

 respiratory 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 respiratory membrane. 

 (Fig. 117.) 



Among the air-breathing Vertebrates there are the added 



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