150 SYDNNY J. HICKSON, M.A., F.R.S. 



Hapalocarcinus has hitherto been detected only in pieces of branching 

 corals of different genera . , . . On all these corals the crabs 

 produce a peculiar excrescence on the twigs (so to speak) of a branch ; 

 these growths .... grow opposite each other in such a way 

 that the crab settled between them is perfectly surrounded, and thus 

 enclosed, in the gall which gradually forms .... A diseased 1 

 excrescence is first produced by the young crab establishing itself 

 between the two branches, and the twig thus originating takes various 

 forms according to the character of the species of coral .... 

 In the first instance the two leaf-like twigs are, of course, far apart, so 

 that the crab could easily get in and out ; but as it does not do this, it 

 is soon so surrounded by the growing together of the twigs that it must 

 remain a prisoner. The creature requires a constant and rapid renewal 

 of the water in the gall in which it lives for respiration .... Since 

 in all the crabs of this group the current of water for breathing enters 

 the body close to the mouth, and passes out again at the hinder margin 

 of the branchial [respiratory] cavity, the stream passing through the 

 gall must always flow in one and the same direction .... The 

 two excrescences on the coral grow together quickest in those spots 

 which are least exposed to the current through the gall .... 

 at length only two fissures .... are left, which plainly show 

 . . . that it is through them that the current for respiration 

 passes .... These two slits remain open so long as the crab is 

 alive ; no living crab is ever found in a closed gall, and they are for the 

 most part perfectly empty." 



1 The excrescences show, as stated above, no signs of disease. 



