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 crinoid 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. Either 

 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. brucei, 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 



