196 Dr. J. S. Thomson on the 



sand and silt, red and brown seaweeds, annelids, lamelli- 

 branchs (such as oysters, barnacles, polyzoa, ostraeods, 

 small algae, and hydroids), and it is impossible to believe that 

 in any of those cases aggressive resemblance would be of 

 any advantage. 



It occurred to me that it was possible that the blue and 

 grey specimens might be located in the higher reaches of the 

 shore and the red on the lower parts, but I found nothing to 

 support this supposition. One may, however, safely say 

 that on the shore the purple and grey specimens predominate 

 in number over the red ; thus out of 341 specimens of 

 Echinus angulosus, observed between high- and low-water 

 marks, 168 specimens were purple, 131 were grey, and 41 

 red. According to Nutting and others blue is a colour which 

 is very rarely present in deep-sea animals. 



I have nothing in support of the surmise that the variable 

 colour of this species may be partly due to the changing 

 conditions of environment which are usually associated with 

 the sea-shore, for specimens from 20-30 fathoms also show a 

 similar variation in colour. At that depth, as on the sea- 

 shore, the purples (so far as my observations went) predomi- 

 nated in number over the red varieties. 



It is much more probable that the variable colour of 

 Echinus angulosus is due to internal physiological processes 

 of the individuals themselves, and it is possible may have 

 some relation to the intensity of light, as held by Doflein for 

 the Decapod Crustacea. According to Doflein feeble light is 

 sufficient for the formation of red pigment, and under the 

 influence of light and still unknown internal processes red 

 may be transformed into yellow or blue pigment, the latter 

 being temporary, visible only when produced in large 

 quantities, but under other conditions is destroyed as soon as 

 formed — in species living in intense sunlight the formation 

 of blue pigment exceeds its destruction. 



The colours of E. angulosus probably have no adaptive 

 significance — in other words, so far as its colour-characters 

 are concerned, these have been evolved apart from the in- 

 fluence of natural selection. 



In Echinoderma the probability is that the pigments of 

 the test and spines are derivatives of " enterochlorophyll," 

 the pigment which occurs in the perivisceral fluid and which 

 by some is held to be a respiratory pigment on account of 

 the changes in colour which it undergoes on exposure to the 

 air, and which we have seen in some cases resembles the 

 colour of the spines. Durham has shown that in Echino- 

 derma pigment occurs in wandering cells which may leave 



