Fauna Associated with Crinoids of Tropical Coral Reef, etc. 79 
too are not easily seen. And lastly, when the dark pigment is uni- 
formly spread over the body of the crinoid this is also the case with 
the alpheid. 
But though this is often true, there are many cases where the resem- 
blance is by no means close and the alpheid is rather conspicuous. I 
am inclined to explain these cases by supposing the alpheids to have 
migrated from some other crinoid at a comparatively recent period. 
So thickly do the crinoids lie in the crevices of the reef, all conceivable 
colour-varieties being herded together without distinction, that it is 
more than likely that an interchange of commensals should occasionally 
take place; for Synalpheus, though tending to become a truly sedentary 
animal, is at times very active and an excellent swimmer. I should also 
like to suggest that the conditions of commensalism do not allow natural 
selection to come into play to any marked extent. The commensals 
are for the most part inclosed by the arms of the erinoid as in a living 
cage. When the water is calm these arms are relaxed and extended, 
but on the approach of an intruder they curl up and so protect the soft 
central disc. It is hardly to be supposed that even a rapacious fish 
would take a mouthful of these hard and unsatisfying arms for the sake 
of the shrimp which lies amongst them. In no case at least which we 
saw were the arms of crinoids mutilated. 
Gamble and Keeble have shown that in the prawn Hippolyte the 
young larva is at first a colourless creature living at the surface of the 
sea, and when this is carried inshore it attaches itself to the first seaweed 
it meets. The skin is in such a sensitive condition that within a week 
a complete resemblance in colour is brought about, whether the seaweed 
background be red, green, or brown. It seems that the resemblance in 
colouring of Synalpheus to its host is a similar phenomenon. But it is 
less perfect because natural selection has not been brought into play 
to the same extent (if at all) as in Hippolyte, whose seaweeds offer it a 
holdfast but not a complete refuge such as the crinoid affords. 
Generally, both members of a pair are similarly coloured. This is a 
phenomenon which may be explained in two different ways. Hither 
mating takes place early, before the pigment pattern has been finally 
determined, and the same causes act equally on both, producing a 
similar pattern, or else there is assortative mating. Possibly the truth 
lies in a combination of both explanations. If so, the exceptions where 
mates are dissimilarly coloured are due to the breaking down of the rule 
of assortative mating or the existence of individuals which are not able 
to assimilate themselves to their background. 
With regard to the habits of S. brucez, it is usual to find the male and 
female lying side by side on the surface of the disc, but when disturbed 
they take refuge between the pinnules or on the aboral surface of the 
arms. They thus move about quite freely, but they can guard against 
forcible detachment by digging the claws of the thoracic legs into the 
