FUNCTIONAL ADAPTATION — - SUBMAMMALIAN PIGMENTS 335 



the frequency of muscular contraction far exceeds that found in any 

 other species. The Hmit is set by the diffusion of oxygen, which is 

 brought directly to the neighborhood of the muscle by the respiratory 

 system without intervention of other carriers. In the case of the 

 lung, however, Roughton {2362-2361^) has considered the velocity of 

 the association reaction in the mammalian erythrocyte and its rela- 

 tion to the time taken for the erythrocyte to pick up oxygen on its 

 way through the lung. In this organ, diminution in the velocity of 

 the association reaction would probably require further anatomical 

 adaptation. In Lumbricu.f, for example, even though the velocity of the 

 oxygen uptake of its carrier may be slower than that of hemoglobin, 

 it seems probable that the development of the anatomy of the 

 primitive respiratory system has lagged behind the development of 

 the pigment. 



11.4. Store or Carrier? 



Since Barcroft's {14-4) considerations of the possible storage function 

 of the hemoglobin in Arenicola, and the demonstration that the 

 amount of oxygen stored is sufficient to last the worm for the period 

 between the tides, a number of organisms have been considered from 

 a similar point of view. In the case of insects, ]\Ialuf (1853) reviews 

 the data up to 1939 and concludes that the pigment functions as a 

 carrier rather than as a store, since the total amount found is rela- 

 tively slight. It seems that in relatively few instances have we 

 enough data from which to draw very definite conclusions on this 

 point. The cyclic changes in the oxygen pressure in the environment 

 are extremely slow in comparison with the cyclic changes in mam- 

 malian muscle. 



The concept of a store to tide the animal over a cyclic change in 

 the oxygen pressure can only be considered a significant evolutionary 

 adaptation if the animal is particularly sensitive to short periods of 

 anaerobiosis. In these lower forms of life, the retention of the more 

 primitive anaerobic metabolism seems a more profitable method of 

 adaptation than the development of an oxygen store. The value of 

 the latter is dependent on the probability that chance variations in 

 the length of time during which the oxygen supply is short will never 

 exceed the oxygen capacity of the store. Ewer and Fox (727) have 

 shown that the chlorocruorin of Sabella acts as respiratory carrier, 

 not as oxygen store (c/. also lJf20). 



