78 Papers from the Department of Marine Biology. 



each side of the carapace, and two on the upper surface of the chela. 

 In addition, each abdominal pleuron is traversed by a short, oblique 

 pigment-free stripe, which may by the encroachment of the pigment 

 become a spot. The relative extent of pigmented and non-pigmented 

 areas is exceedingly variable and corresponds roughly to the depth of 

 colouration of the host which the alpheid inhabits. Individuals which 

 lodge upon a light-green coloured crinoid have only thin red or purple 

 lines of pigment, the rest being unpigmented; in those which are asso- 

 ciated with a dark green or black host the pigment may be spread over 

 the whole of the dorsum. 



In the vast majority of cases a pair, male and female, of alpheids is 

 found on each comatulid. In a few cases there appeared to be only a 

 single lodger, but then its small size seemed to show that it was too 

 young to have acquired a mate. In most cases the pair are similarly 

 coloured; rarely, however, there is a difference, and I will mention one 

 case in which one member was marked with very definite and fairly 

 wide stripes of dark pigment, while the other was uniformly covered by 

 red pigment. 



There are two factors in colour variation. The one is the character 

 of the pigment, which ranges from a clear red to a purple so deep as to 

 appear black. The second depends on the area of the body covered by 

 the pigment. The former factor seems to depend very largely on the 

 species of the host. Synalpheus brucei is found (though more rarely) 

 on a second host, Comatula purpurea, in which reddish pigment pre- 

 dominates, and here the crustacean was sometimes observed to show a 

 red pigment which covers the whole body and thus matches the host. 

 But sometimes purple and white striped individuals are found on Coma- 

 tula purpurea, thus transgressing the apparent needs of the species. 



Among the commensals of C. annulatum, too, the pigment may be red 

 or red brown. But this is only so in the cases where the coloured stripes 

 are narrow. Where they are broader the pigment is invariably darker. 

 The chief variations of colour, then, may be tabulated as follows: 



Uniformly red-brown In C. purpurea. 



With narrow red stripes In light-green varieties of C. annulatum. 



With purple stripes or uniform dark colouration . . In dark-green varieties of C. annulatum. 



The purple pigment would seem to be either something additional or 

 a more complex product of the red pigment. In alpheids which were 

 preserved in formalin and glycerin to keep the original colour as far as 

 possible, the purple was speedily lost, leaving such a red as occurs in 

 the lighter-coloured forms. 



Synalpheus brucei thus exhibits a marked protective resemblance to 

 its host. Against the very light-coloured crinoids the almost colourless 

 alpheid is quite inconspicuous. The striped specimens are found upon 

 crinoids where bars of light and dark pigments alternate, and so they 



