VI COLOURS OF CRUSTACEA AND ECHINODERMA 125 
the Challenger remarked on the fact that a bright 
red pigment (which he called crustaceorubrin) should 
occur in small surface forms like Daphnia , and 
then reappear in the ocean depths. The probability 
is, however, that this pigment is very widely dis¬ 
tributed in the Crustacea, and that its apparent colour 
depends upon the conditions under which it occurs. 
As to the simultaneous occurrence of red and 
yellow lipochromes in the Crustacea, there is no 
doubt that by various agents, and especially in the 
presence of heat, both red and yellow pigments can 
be extracted from the skin and shell of the lobster 
and crayfish. I am, however, much inclined to doubt 
the existence of both these pigments in the living 
condition. Certainly the yellow, if it exists, has 
practically no effect upon the coloration. The epi¬ 
dermis of the lobster yields to water a beautiful 
bright red solution, with no trace of orange, and does 
not on filtering leave behind any orange or yellow 
pigment. Lipochromes as such are of course not 
soluble in water, but the red ones dissolve very 
readily in solutions containing albumen. 
The red lipochromes, when in solution in water 
containing proteid, are precipitated with the proteid 
on boiling, but if alkali be added to the solution, 
alkali-albumen is formed, and they are not precipitated 
even on boiling. The fact that the red lipochromes 
thus form compounds with alkalies which are soluble 
in albuminous solutions, is one of considerable in¬ 
terest to which we shall recur. 
2. The Soluble Blue Pigments .—These unstable 
pigments constitute the cyanic series of Pouchet, 
the lipochromogens of Krukenberg. They occur 
