VI COLOURS OF CRUSTACEA AND ECHINODERMA 123 



remarkable in an animal generally distinguished as 

 compared with its allies by a marked deficiency of pig- 

 ment. In general it may be said that the Crustacea 

 exhibit a marked tendency to vary in colour, and 

 especially to oscillate between shades of blue and 

 green on the one hand and of red and yellow on the 

 other ; they at the same time often exhibit much 

 constancy in detail, and in deep-sea forms the blue 

 and green colours tend to disappear. 



So far we have considered the two sets of colours 

 as if they were entirely distinct from one another, but 

 it is familiar to all that the lobster turns red when it 

 is boiled ; in other words, the action of a large 

 number of agents, such as heat, alcohol, ether, dilute 

 acids, etc., upon the blue or green series is to convert 

 them into the red. The blue or cyanic series occur, 

 as we have seen, in solution, while the reds occur in 

 fixed anatomical elements — the chromatophores. 

 This difference in distribution has caused many to 

 contrast the two sets sharply, and to suppose that 

 the reddening of the lobster is due to the total 

 destruction of the blue pigment, which allows the red 

 to become visible. By others, and notably by Kruken- 

 berg, it has been maintained that the blue pigment 

 is a lipochromogen, which very readily undergoes 

 decomposition and then becomes converted into a 

 lipochrome. The following description is based on 

 observations of my own in the case of Astacus, 

 Homarus, and Nephrops ; for details reference should 

 be made to my paper on the subject. 



