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 Pterwides 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 purple, 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. 
