460 Mr Potts, The gall-forming Crab, Hapalocarcinus. 
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The gall-forming Crab, Hapalocarcinus. (Preliminary Note.) By . 
F. A. Ports, M.A, Trinity Hall. | 
[Read 4 May 1914.} 
Hapalocarcinus is a genus of Brachyrhynch crabs very small in ( 
size and profoundly modified owing to the fact that they pass the « 
greater part of their lives confined in small cavities in coral, 
colonies. At an early age the crab settles between two branchlets, : 
usually terminal, and so influence their further growth that they ) 
broaden and, later, unite to form the so-called gall, a lenticular or j, 
spherical structure, about the size of a hazel nut. Within this is | 
the living chamber of the crab which communicates with the 
outside water by a series of apertures. Information about the 
biology of this form is however particularly scattered and incom- | 
plete, and while the structure of the adult female is fairly well | 
known and the systematic position has been adequately discussed | 
by Dr W. T. Calman*, the male has remained undiscovered up till | 
the present. I am greatly obliged to Dr Calman who directed my ! 
attention to this teresting creature and the need for a connected | 
account of it. 
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I found Hapalocarcinus during the month of October, 1913, | 
existing in great numbers on the reefs of Murray Island at the | 
north end of the Great Barrier Reef of Australia. It here forms | 
galls on two species of branching Madreporarian corals Pocillopora 
caespitosa and Seriatopora hystrix, both belonging to the same } 
family, the Pocilloporidae and characterised by dichotomous : 
branching. Both are widely distributed and dominant forms but | 
the former is the favourite host of Hapalocarcinus and in the still | 
waters which cover the inner reef there is hardly a colony which | 
does not bear at least one of the galls, while some show nine or ° 
ten in various stages of development. The ease with which the » 
growth of the colonies can be modified by external agencies | 
accounts for the attraction which Pocillopora possesses for the - 
gall-forming crab. No coral shows more variation under different — 
environmental conditions. Seriatopora shows less power of response — 
to the influence of currents. Galls are by no means so common as 
in Pocilopora for they represent a much greater interference with — 
the type of branching. ‘The remarks which follow are then con- 
fined to Pocillopora, the abundance of the material and the 
uniformity of development giving great facility to study. 
It the first place it may be stated that though the galls 
themselves vary greatly in development, there is a general relation 
* W. T. Calman, Trans. Linn. Soc. Zoology, 2nd Ser. Vol. vu. Pt 1. p. 43. 
