19J: Dr. J. S. Thomson on the 



notes in regard to the solubility of the pigments may be 

 added. On adding fresh water to a mixture of the spines 

 and tube-feet of Echinus angulosus, the following results 

 appeared almost immediately : — Green produced a pink 

 solution; grey, pink solution (very similar to that of the 

 perivisceral fluid) ; red, very slightly tinted pink solution ; 

 purple, slower appearance of a pink solution. The perivisceral 

 fluid is pink at first, but becomes darker on exposure. The 

 following solutions were obtained after the spines and tube- 

 feet had been allowed to remain in the water for fourteen 

 days: — Green, yellowish-green solution; grey, similar to 

 that of green ; red, slightly pink ; purple, brownish to 

 purple; the solutions having a disagreeable odour. 



Effect of reagents on the spines (apart from the tube-feet) : 

 solutions of the following colours were obtained : — 



Ether : red spines, yellow solution ; light pink, yellow ; 

 lilac, very faintly tinted solution. 



Ammonium chloride: red spines, a pink solution at first, 

 which becomes darker, and is after two days a reddish 

 brown. 



Glycerine : red spines, a red solution quickly obtained ; 

 purple spines, a slightly tinted purple solution ; in the case 

 of the red spines the solutions became much intensified in 

 colour on longer solution, but this was only extremely 

 slightly the case with the purple. 



Caustic potash : on red spines practically no result at first, 

 later a yellowish-red to red solution. 



As a. whole, one may say that the colours of the spines 

 are very unstable and are slightly soluble in such reagents as 

 glycerine, ether, ammonium chloride, and perhaps caustic 

 potash. 



In regard to the meaning of the coloration of the Echino- 

 derma there may be great diversity of opinion. The custom 

 of propounding ingenious explanations as to the meaning of 

 colour from the utilitarian standpoint is too prevalent. In 

 regard to the species Echinus angidosus, one explanation of 

 the colour must be at once excluded — namely, sexual coloration. 

 Among marine animals we find a number of cases of specific 

 variation in colour which can be explained in part along the 

 lines of protective coloration ; thus, Planes minutus among 

 the Brachyura shows at least five variations of colour, all of 

 which are in harmony with the tints of the Sargasso weed. 

 I am unable to apply the principle of protective coloration to 

 the case of Echinus angidosus. It is true that the red variety 

 is sometimes found at depths at which in ordinary circum- 

 stances it may, from the diving experiments of Fol at Nice, 



