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 



