Body Fluids 111 



otic notch in Amphibia corresponds closely to the opercular flap 

 in fishes" (Gregory, 1933). Morris (1892), Spengel (1904), Bal- 

 lantyne (1927), Goodrich (1930), and others have discussed the 

 origin of lungs. 



Pike (1924) has reviewed the changes in vertebrate respiratory 

 mechanisms. The appearance of lungs necessitated the development 

 of new respiratory muscles. Coordination between swallowing and 

 breathing was necessary. At first the body wall functioned for 

 breathing. A reptile could live out of water. It might swallow in 

 a leisurely way and breathe at the same time. A mammal with a 

 diaphragm did even better. New nervous and muscular mechanisms 

 made possible the methods of breathing which are now correlated 

 with the rapid metabolism of homoiothermic animals. 



Body Fluids 



"The first living forms had the sea for their environment. Every 

 cell, doubtless, came into contact with this fluid which was at the 

 same time the source of its food and of its oxygen supply. As cells 

 began to be associated in smaller or larger masses, channels were 

 left between them through which the water of the sea might find 

 passage. Animals a little farther along the scale of development 

 shut off their body cavities, vascular and otherwise, from direct 

 communication with the sea, but did not succeed in freeing the cells 

 from the necessity of getting their food and oxygen supplies by 

 diffusion from a solution. The fluid shut within the animal body 

 furnished the immediate environment of the cells; it took the place 

 of the sea in the economy of the organism. It has been seen that 

 the fluid becomes more and more complex in structure as one passes 

 in review from lower to higher forms. And while the internal fluid 

 sea, bathing the individual cells, has become in some respects more 

 complex and able to play a greater variety of functions in the life 

 of the organisms, there are certain features in which it harks back 



