412 



THE AMEBIC AN NATURALIST 



[Vol. LIV 



mentary accumulation of some metabolic waste. This view in 

 itself does not preclude the possibility of an adaptive deter- 

 mination of the pigmentation ; it would be curious indeed if an 

 internal coloring-matter were not the result of metabolism. In 

 order to remove the possibility of an explanation for the colora- 

 tion in the customary terms of adaptation, it must be shown that 

 the pigmentation in question is an unconditioned result of 

 metabolic processes— unconditioned, that is, by the "need" for 

 concealment and the like. It is, therefore, important to observe 

 that: (1) the degree of pigmentation increases with age; (2) 

 the variously colored individuals live side by side; (3) the tilt- 

 ing of the body exhibited during burrowing, and the exposure 

 of the posterior margin of the body owing to the incompleteness 

 of this act, are not compensated by counter shading — the ven- 

 tral surface is more darkly colored than the dorsal; and (4), the 

 region known to be differentially exposed in this way is found 

 actually to bear evidence of damage, in a goodly proportion of 

 individuals. The exact origin of these injuries remains obscure, 

 but is not of primary importance here. 3 



The general physiology of pigmentation in the sand-dollars 

 and sea-plates, and the possible evolutionary significance of the 

 growth changes in body-form noted in this paper, should be 

 made the topics of further studies. 



Dyer Island, Bermuda, 1918. 



3 I have good reason to believe that the injured mellitas were not dam- 



