Nos. 622-623] SHORTER ARTICLES AND DISCUSSION 555 



from bright green to reddish purple at a hydrogen-ion concen- 

 tration of about 7.5, the ' ' echinochrome " not becoming orange 

 until a much greater acidity is reached. This pigment becomes 

 blue at an alkalinity of about p 7/ =8.2; in the ovary it is red- 

 dish purple, but it is not in solution. 



The interest of these facts lies not so much in their supplying 

 some additional instances of the elaboration of similar (or even? 

 identical) substances — conspicuously pigments — by animals re- 

 lated in descent, but in the evidence which is afforded regard- 

 ing the reaction of intracellular fluids. Numerous cases illus- 

 trating the former point are available from among echinoderms, 

 molluscs, tunicates, and so forth, and these cases have a certain 

 importance for the general theory of animal coloration. But I 

 am here chiefly concerned to point out that, if the alkali-greening 

 substance present in 31. sesquiperforatus is closely similar to 

 that produced in the tissues of other clypeastroids, unequivocal 

 statements as to the occurrence of greenish hues in living "sand 

 dollars" may contain a suggestion as to a possible mode of origin 

 for certain known geographical color differences in echinoderm 

 species. I have not been able to find such references, but the 

 point is worthy of further study. The evidence afforded by 

 intracellular substances capable of behaving as indicators of 

 acidity shows plainly that the tissues of marine animals are 

 much more acid (less alkaline) than sea water. In M. sesqui- 

 perforatus the green color is produced when the tissue fluids 

 become more alkaline than they customarily are; thus the in- 

 tegument, when injured or killed, becomes permeable to sea 

 water and assumes a green hue. Healthy individuals for ex- 

 perimental work may. incidentally, be selected (at Bermuda) 

 by the absence of green areas upon the test ; the readiness with 

 which greening is induced indicates the degree of care which 

 must be employed in handling some marine animals. If by 

 some means, for example, by higher temperature, the tissues of 

 a Mellita population in a warmer sea were constantly main- 

 tained at a higher alkalinity than those at Bermuda, they might 

 normally appear somewhat greenish in color. The normal varia- 

 tions in the coloration of Chromodoris zebra, a nudibranch con- 

 taining an indicator favorable for such observations, strongly 

 suggest that such differences in the reaction of intracellular 

 fluids (not necessarily of the protoplasm) are entirely possible 

 (Crozier, 1916). Whether or not comparable changes may be 



