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. 
