Hapalocarcinus, the Gall-forming Crab, etc. 43 



in the fully formed gall. But as I have indicated above, in Pocillopora, 

 too, there is really an increased branching masked by immediate 

 fusion of the branches, so that the process of gall formation is not 

 essentially different in the two genera. 



It is rather difficult to follow the account of gall formation in Seria- 

 topora given by Semper in "Animal Life." This book represents a 

 course of lectures given at the Lowell Institute in Boston in 1877 and 

 I am not able to say whether a German edition of the book was ever 

 published. The English work has, however, suffered very badly at the 

 hands of the translator and it is sometimes difficult to determine what 

 Semper intended to say. Take for instance the following sentence: " A 

 diseased excrescence is first produced by the crab establishing itself 

 between two branches, and the twig thus originating takes various 

 forms according to the character of the species of coral." What seems 

 to be meant is that when the young crab establishes itself between two 

 branches it modifies and stimulates the growth of each of them and the 

 modified branches (twigs) take on various forms according to the species 

 of coral which is in question. This is of course perfectly true and cor- 

 responds to what I have said in the preceding accounts. The idea that 

 the growth brought about by Hapalocarcinus is pathological is given 

 in the application of the ungraceful term of "diseased excrescences" 

 to it. This is entirely unwarranted, since similar modifications affect- 

 ing the whole colony take place under the influence of wave action. 

 There is, moreover, no reason for calling the crab a parasite, since it 

 does not live upon the tissues of the coral; but even Caiman falls into 

 this error. 



Semper, after describing the process by which an open is converted 

 into a closed gall, makes the following remarks: 



"The creature requires a constant and rapid renewal of the water in the 

 gall in which it lives, for the purpose of respiration; at first the water finds a 

 free passage on all sides, but when the two twigs have bent over towards each 

 other, the space through which it can find entrance and exit must grow nar- 

 rower and narrower. Moreover, from the structure exhibited by galls broken 

 off from the coral, it may be concluded with certainty that the crab moves 

 about very little in the cavity, for otherwise we should not find the very 

 distinct scars which arc evidently produced by continual scratching in one spot. 

 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 cavity, the stream passing through the gall must always flow 

 in one and the same direction. The results are easily recognisable in the half 

 or wholly closed gall. The two excrescences on the coral grow together 

 quickest in those spots which are least exposed to the current through the gall ; 

 there also they first come into contact, till at length only two fissures, more or 

 less wide, arc left, which plainly show, by their position opposite to each other, 

 that it is through them that the current for respiration passes; one fissure 

 serves for the influx, the other for the exit, of the water. 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." 



