VI COLOURS OF CRUSTACEA AND ECHINODERMA 119 
the smaller and more delicate forms. The resem¬ 
blances between shrimps, sand-hoppers, shore-crabs, 
etc., and the localities which they respectively haunt 
are too patent to need emphasis, and phenomena of 
this nature are exceedingly common in the group. 
In many cases the colours vary in harmony with 
the environment, but this can only occur in forms in 
which the epidermis is the important agent in colora¬ 
tion, and is then due to the sensitiveness of the 
chromatophores. We find this in the shore-crab in the 
young stage, and in many of the small sessile-eyed 
Crustacea, such as Idotea and Caprella. The prawn 
Hippolyte, according to M. A. E. Malard, is green on 
green seaweed, brown on Fucus , red on red seaweed, 
and transparent among Antennularia and Sertulana. 
Experiment in artificial environment showed that 
this form is red in darkness, emerald-green in bright 
light, and brown in semi-obscurity, which seems to 
prove that it is the intensity of light rather than its 
colour which is of importance in effecting colour- 
change. Pouchet found that prawns (e.g. Leander 
serrator ) turned brownish-red in vessels with black 
bottoms, and if then transferred to white vessels 
became yellow, passing through a stage in which 
they were bright blue. Pouchet stated that the 
brownish-red colour was due to a combination of a 
bright red colour due to chromatophores and a 
soluble blue pigment. When the prawns were re¬ 
moved to a white dish, the chromatophores slowly 
contracted and allowed the blue colour to become 
apparent. As the contraction proceeded the blue 
pigment gradually disappeared, and the ultimate 
yellow colour was produced by the contracted 
