84 



CORALS 



prisoned for life (Fig. 35) 



called a friendh' association with a little crab (Hapalocar- 

 cinus) . 



^^'llen this crab is very small it settles down in the fork 

 between two young branches and, by some kind of continuous 

 irritation or stimulation, causes each branch to divide into 

 a number of lateral but anastomosing branch lets which, 

 spreading out on each side of the fork like a fan, eventually 

 converge above and form a cage in which the crab is im- 



In some specimens a large num- 

 ber of these crab cages may be 

 seen, and so far as can be judged 

 by appearances they do not seem 

 to interfere with the general well- 

 being of the colony as a whole. 

 It may seem to be a bold asser- 

 tion to make, that imprisonment 

 for life is beneficial to any li\'ing 

 creature, but as the adult female 

 Hapalocarcinus is never found 

 anywhere except in one of these 

 cages it may be presumed that, 

 if she has a mind, she prefers it. 

 At any rate it is certain that 

 confined to this prison she can 

 obtain sufficient food for her 

 nourishment and can successfully 

 reproduce her kind. 



The special point of interest 

 for the student of corals to consider is why these crab cages 

 are so frequently found in these two genera, Pocillopora 

 and Seriatopora, and not in others. The only recorded 

 instance of a crab cage of this kind on a coral of another 

 genus is in a specimen of Millepora from the West Indies, 

 now in the Public Museum at Liverpool, but they are not 

 known to occur on any species of Madrepora, Oculina, or 

 other corals with a similar method of branching. Is there 

 some special scent or flavour in the Seriatoporidae which 

 attracts the crabs to these corals ? This is a question to 

 which no satisfactory answer can be given at present. 



Fig. 35. — An example of Seria- 

 topora, ill which some of the 

 branches have coalesced to form 

 a gall for the crab Hapalocarcinus. 

 Nat. size. 



