Fauna Associated with Crinoids of Tropical Coral Reef, etc. 93 



In conclusion, this species is quite distinct from, though related to, 

 Poly noc crinoidicola, a species found on various unnamed crinoids in 

 the Maldives by Professor Gardiner and described by me in 1910. The 

 differences lie in the shape of the head and the tentacles and in the neu- 

 ropodial setae, but the general facies and the character of the elytra and 

 the shape of the parapodia and of the dorsal setae are similar. The 

 specimens of crinoidicola are spoken of in one case as coming from a 

 black crinoid and being themselves black when alive, though the pig- 

 ment has dissolved out in alcohol. In other cases where the colour of 

 the live animal and the host were not stated, the specimens were dark 

 red. It seems certain, then, that the same type of colour resemblance 

 occurs in the two species. In Torres Straits, too, I found the com- 

 mensals always of a dark, almost black appearance and frequenting 

 dark-coloured crinoids. They must, I think, be absent or rare on the 

 lighter coloured hosts. 



The only other reference of which I have knowledge to commensalism 

 between a polychaete (?) and a crinoid is in Alcock's "A Naturalist in 

 Indian Seas/' p. 113, where he mentions a "sea-lily (Actinometra) 

 striped in alternate bands of yellow and purple, on whose fronds simi- 

 larly striped crustaceans live without fear of detection;" which in some 

 places also gives "secure shelter to seaworms, banded yellow and 

 purple like itself." This indicates a much wider range of pigmentation 

 than in the present case. The banding is probably longitudinal, as in 

 all other cases of commensalism. 



THE COLOURATION OF THE MYZOSTOMIDS AND ITS RELATION TO 



THAT OF THEIR HOSTS. 



From time to time remarks have been made on this subject. Semper* 

 relates that he found his M. tuberculosum, which is spotted with red and 

 yellow, only on variegated Comatulas, while the uniformly coloured 

 M. cirriferum is found only on the red Comatulas, which it matches in 

 colour. Von Graff ,f however, remarks that though M . glabrum has a 

 number of pronounced colour varieties, and the host Comatula europcea 

 s. mediterranea varies within the same limits and almost with the iden- 

 tical shades of colour, yet his experience leads him to deny that any 

 such definite mimicry relation exists between crinoid and myzostomid. 

 A special investigation of over 200 comatulids showed that blackish, 

 yellow and white myzostomids were just as frequently to be found on 

 red Comatulids as blood-red myzostomids on variegated comatulids, 

 and that the two kinds specially mentioned by Semper are uniformly 

 distributed on the different colour varieties of the crinoid. 



*Semper, Zur Anatomie und Ent \vickelungsgeschichte der Gattung Myzostoma. Zeita. f. 

 Zool. Bd. ix, pp. 48-65, 1858. 



tvon Graff, Das Genus Myzostoma. p. 77, Leipsig, 1877. 



