of the Fauna associated with Orinords 61 
__ The majority of the crinoids are dark green or black in colour 
and most of the commensal Alpheids are completely covered by 
a black or brownish-purple pigment. But often the pigmented 
dorsum is traversed by longitudinal stripes which are free from 
pigment, one down the middle line and two or three on each side. 
The relative extent of pigmented and nonpigmented areas is 
exceedingly variable and corresponds roughly to the coloration 
of the host which the Alpheid inhabits. Individuals lodging on 
erinoids whose general hue is light green have only thin purple 
stripes the rest of the body being unpigmented. In those members 
of the species S. Brucet which frequent a second host, Comatula 
pectinata, in which a bright red pigment predominates, the crustacean 
was also observed to show a red pigment covering the whole body. 
The range of colour variation is thus in S. Bruce: limited between 
these two related types the red and purple; but by an adjustment 
of the relative extent of the bands of colour the animal may become 
Meonspicuous upon the host whatever the colour scheme of the 
latter may be. 
In most cases both commensals on any crinoid are similarly 
coloured: rarely however there is a marked difference. I may 
mention one case in which the one member was marked with 
very definite and fairly wide stripes of dark pigment (the inter- 
mediate non-pigmented areas being prominent) while the other 
was uniformly covered by red pigment. I am inclined to explain 
this and all cases where the Alpheid is rather conspicuous upon 
its host (as occurs in a certain proportion of individuals) by supposing 
it to have migrated from some other crinoid at a comparatively 
recent period. So thickly do the crinoids lie in the crevices of the 
reef, all conceivable colour-varieties being herded together without 
distinction, that it is more than likely that an interchange of com- 
mensals should occasionally take place for Synalpheus, though 
tending to become a truly sedentary animal, is yet very active 
and an excellent swimmer. 
___Indiscussing generally the protective resemblance of Synalpheus 
to Comanthus a.com parison may profitably be made with the classical 
case of Hippolyte varians described by Gamble and Keeble. The 
young Hippolyte is free swimming and colourless but it becomes 
Virtually a sedentary animal anchoring itself to a seaweed in the 
Laminarian zone on which it finds both food and shelter. The 
‘prawn has the power of forming red, yellow and blue pigments 
and by altering their relative proportions in the chromatophores 
1é can acquire a green, brown, blue or red ground colour and is 
thus able to adapt itself to the varied colours of the seaweeds and 
hydroids. The pigment may be laid down in longitudinal stripes 
or horizontal bars and in this way a colour scheme can be formed 
Beching whatever seaweed the prawn shelters in. In early life 
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