74 Papers from the Department of Marine Biology. 



In the majority of cases the association is of a vaguer nature, and 

 while the associated animal gains protection it obtains its own food- 

 supply. How numerous such cases are in tropical seas may be seen 

 from the following passage : 



"We noticed numerous other animal partnerships, which might have been 

 cases of commensalism but were more probably merely one-sided adaptations 

 of one animal standing in need of protection to another animal capable of 

 affording the required protection without any expenditure of effort. For 

 instance, a very common branching zoophyte of this region is Spongodes 

 pustulosa (or some very closely related species), a creature near akin to the 

 'dead men's fingers' of British seas. It looks like a small 'run to seed' cauli- 

 flower, of which the individual florets are of a bright pink colour. Hidden 

 among its branches we found no less than four small species of crustaceans (an 

 Alpheus, a Galathea, a Porcellana, and a rare little spider crab known as 

 Hoplophrys oatesi), all of which, in life, are greyish white, with bright pink 

 spots, so that they are perfectly invisible so long as they remain quiet in their 

 living refuge. Another zoophyte that we often dredged was Pterceides elegans 

 (or a species intimately close to it), one of the seapens, of a grey colour pro- 

 fusely marked with little blackish rings. In its leaves three small species of 

 crustaceans are accustomed to hide, all of which are coloured and spotted 

 exactly like the living citadel in which they dwell. I have already mentioned 

 the sea-lily (Actinometra) , striped in alternate bands of yellow and puVple, on 

 whose fronds similarly striped crustaceans live without fear of detection ; here 

 we found the same sea-lily giving secure shelter to sea-worms, banded yellow 

 and purple like itself." (A Naturalist in Indian Seas. A. Alcock, London, 

 p. 112, 1902.) 



The association last mentioned in this passage, that between stalkless 

 crinoids and a multitude of smaller invertebrates, forms the subject of 

 this paper. To those who only know the species of Antedon found in 

 our own British waters, the wealth of numbers and the riot of colour in 

 the crinoid fauna of a tropical coral reef is a remarkable revelation. 

 In October 1913, during my visit to Murray Island, I was able to 

 observe this fauna under the best conditions. The commonest species 

 there is the form Comanthus annulatum. (Bell), remarkable for its extra- 

 ordinary range of colour variation from very light-coloured individuals 

 (in which white, light green, yellow, and grey mingled in the colour 

 scheme) to others which are entirely dark green or black. In the 

 shelter of its arms live commensal forms belonging to many groups of 

 marine invertebrates, and generally speaking they possess a type of 

 colouration which makes them inconspicuous upon the host and so 

 varies with the colour of the host. The fact that such a relation exists 

 between crinoids and such animals as alpheids, galatheids, and worms 

 has been pointed out by Dana, Haswell, and Alcock, but I think the 

 circumstances warrant the publication of a more minute though still 

 very incomplete study of this curious phenomenon. 



