90 Papers from the Department of Marine Biology. 



SUMMARY. 



The object of this paper is to point out that of the large number of 

 species belonging to many phyla which habitually shelter amongst the 

 branches of the crinoids of tropical reefs, by far the greater proportion 

 exhibit colour resemblance to the host. As the colour scheme of the 

 host is often extremely variable (e. g., Comanthus annulatum), that of 

 the commensal, in some cases at least, varies too, but some commensals 

 seem to be restricted in their response to the stimulus which causes this 

 colour resemblance. Thus among the Crustacea, in Synalpheus all 

 stages of variation are met with, according to the individual inhabited, 

 between a pale form with very narrow stripes of pigment, to an extreme 

 form totally covered with dark pigment; while in Cirolana, on the 

 other hand, the individuals associated with even the darkest crinoids 

 possess only insignificant lines of pigment on the otherwise totally 

 white body. In the latter case there is, then, no protective resem- 

 blance, although we witness the incipient stages of its establishment. 

 Other forms like the polychset, the amphipod, and the brittle-star, 

 Ophiomaza, give rise to dark varieties only and occur upon dark-green 

 crinoids. 



The following represents in a tabular form the results of the obser- 

 vations : 



Commensal. 



Pigments developed. 



Host and colour variety of host. 



Synalpheus brncei . 



Purple-black 



Synalpheus brucei. 

 S. comatularum. . 



Cirolana lineala . 



Red 



Brownish purple, purple 

 and white. 



Red or purple in faint 

 thin lines, rest white. 



Cyclotelson purpureum. 

 Pontoniopsis comanthi . 



Periclimenes spp 



Ophiomaza cacaotica. . . . 



Ophioniaza cacaotica var. picta . . . 

 Ophiomaza obscura 



Polynoe minuta 



All purple 



Bright green and black or 



yellow and red-brown. 



Red and blue-purple 



Dark purple, brown or 



black. 



Black and white , 



Dark brown and dull 



yellow. 

 Dark brown or black . . 



All colour varieties of Coman- 

 thus annulatum and some 

 other crinoids. 



Coraatula purpurea. 



All colour varieties of Coman- 

 thus annulatum and some 

 other crinoids. 



All colour varieties of Coman- 

 thus annulatum and some 

 other crinoids. 



Dark varieties of C. annulatum. 



Light (green or yellow) vari- 

 eties of C. annulatum. 



Dark varieties of C. annulatum. 



Dark varieties of C. annulatum 

 (one exception). 



Dark varieties of C. annulatum. 



Dark varieties of C. annulatum. 



Dark varieties of C. annulatum. 



