VI COLOURS OF CRUSTACEA AND ECHINODERMA 121 
phenomenon is not absolutely dependent upon a 
single environmental factor for its manifestation. 
The other suggestion that the absence of blue or 
green colours among deep-sea Crustacea is due to 
adaptation, because such colour would be invisible 
in the dark abysses, is even less satisfactory as a com¬ 
plete explanation of the common colour-variations 
of the Crustacea, for there are many facts besides 
those mentioned which suggest that there is a 
necessary relation between the different colours. 
The constant tendency visible in the group to 
oscillate between red and blue can hardly be ex¬ 
plained throughout by adaptation. As facts are 
always more convincing than general statements, 
we may add to the examples mentioned above an 
account of the seasonal colour-change in Holopedium 
gibberum , one of the Daphnids, which shows clearly 
the relation existing between red and blue. 
This little form was studied by Professor Anton 
Fritsch in the ponds of Bohemia, and was found to 
be colourless in winter. Towards the end of May 
the first trace of bright colour appeared in the shape 
of a diffuse rose-colour, accompanied by a blue tint 
in the neighbourhood of the mouth and two narrow 
blue stripes at the sides of the abdomen. The diffuse 
rose-colour was most common in dead individuals 
lying at the bottom of the pond, but it also occurred 
in living forms. 
By the end of July brightly coloured individuals 
were very numerous ; in these the under surface of 
the food-canal from the mouth to the end of the 
abdomen was coloured blue, except at the base of the 
third pair of legs, where there was a cluster of bright 
