352 ELEMENTS OF BIOLOGY 



for their special methods of locomotion and special habitats and 

 possess structures that represent wide alterations from a basic type. 

 For example, the bird body contains no structure that is more 

 strikingly fitted for its function than is the human hand. 



Special Types of Adaptive Structures. Although the adap- 

 tive features of the bird, of the human hand, of the frog and fish, 

 and so on, are as illustrative as any, yet the animals are so well 

 known that they arouse no particular interest and are accepted as 

 commonplace. One's curiosity is usually much more stirred when 

 attention is directed to exotic and bizarre forms that are not com- 

 monly met with, and to organs that are either lacking in Man or 

 else are very different from their human analogues. Anything like 

 a complete account of such extraordinary animal features could not 

 be undertaken here; nor would it be desirable, for our chief objec- 

 tive is the exposition of principles that are applicable to all animals. 

 However, some peculiar adaptations will be given brief attention 

 for they show how certain fundamental properties of protoplasm 

 have been exaggerated and exploited to constitute organs that have 

 peculiar properties. 



The Gas Bladder of Fishes. If one opens the coelcm of 

 a perch or other common teleost fish, a long, fairly thick-walled, 

 membranous sac will be found along the dorsal wall of the cavity. 

 In life it is filled with gas, usually chiefly oxygen, and serves as an 

 organ of buoyancy. The gas is secreted from the blood by a small 

 gland in the wall of the gas bladder; thus the hemoglobin of the 

 fish blood serves to carry oxygen not only for the respiration of the 

 tissue cells but also for the filling of the gas bladder. Since the gas 

 in the bladder is nearly pure oxygen, it is clear that its walls are 

 impermeable to this gas, else it would diffuse back into the blood 

 stream, the oxygen concentration in the bladder being higher than 

 that of the blood. It appears that the amount of gas in the bladder 

 varies with the weight of the fish and with its depth in the water. 

 If a weight is attached to a fish the animal soon becomes accus- 



