28 ELEMENTARY ZOOLOGY 



being the means by which oxygen and food are carried 

 to all growing or working parts of the body, and by 

 which carbonic acid gas and other poisonous waste 

 products are brought away from these parts. But this 

 function differs somewhat in its performance in the two 

 animals, and the organs which perform the function are 

 correspondingly modified in structural condition. 



Both toad and crayfish have organs for respiration, that 

 is, for breathing in oxygen and breathing out carbonic 

 acid gas. But the toad takes its oxygen from the 

 atmosphere about it; its respiratory organs are the 

 lungs, the sac-like tube leading to the mouth, and 

 the external openings for the ingress and exit of the 

 gases. The crayfish, living mostly in the water, takes 

 its oxygen from the air which is mechanically mixed 

 with the water. Its respiratory organs are its gills. 

 There is a great difference, apparently, in the structural 

 .conditions of the organs of respiration in the two animals. 

 As a matter of fact the difference is less great than, at 

 first sight, appears to be the case. The lungs of the toad 

 are composed primarily of a thin membrane, in the form 

 of a sac, richly supplied with blood-vessels. Air is 

 brought to this thin respiratory membrane and by osmosis 

 the oxygen passes through the membrane and through 

 the thin walls of the fine blood-vessels, and is taken up 

 by the blood. At the same time the carbonic acid gas 

 brought by the blood to the lungs from all parts of the 

 body is given up by it and passes through the membranes 

 in order to leave the body. The air comes in contact 

 with the respiratory membrane (which is situated inside 

 the body) by means of a system of external openings and 

 a conducting chamber, and by these same openings and 

 chamber the carbonic acid gas leaves the body. In the 

 crayfish the gills are nothing else than a large number of 

 small flattened sacs each composed of a thin membrane 



