GALLS ON CORALS. 217 



carcinus marsupialis, whicli liad been discovered in the Pacific 

 Ocean by Dana, in the course of his great voyage under the 

 command of Wilkes. Irrespective of other peculiarities, this 

 was distinguished from all other crabs by a remarkable pouch 

 in which the female carries the young, formed by a prolonga- 

 tion of the lateral plates of the abdomen. Subsequently Heller 

 described another species of crab from the Red Sea, under the 

 name of Cryptochirus coralliodytes, of which the female has 

 egg-pouches similar to those of the other genus, but the form of 

 the body is otherwise quite different. While the general form 

 of Hapalocarcinus is lenticular, Cryptochirus has a thorax of 

 perfectly cylindrical shape, with a head terminating obliquely, 

 so that it strikingly resembles several of the cylindrical wood- 

 boring beetles. The singular mode of life of these crabs was, 

 however, unknown to both these naturalists. 



Fig. 64.— The crabs forming galls on coralg. a, Cryptochirus fmale) ; 6, Coralliodytes 

 (female) ; c, Hapalcccvcinus marsupialis (female). 



As 1 was able to study both species alive in the Philippine 

 Islands, I will here give my observations in detail. 



For both of them an association with living Corals is indis- 

 pensable, and the influence of the Corals on the Crabs is as 

 direct and i mportant as that of the Ci-abs on the Corals. Hapalo- 

 carcinus '"^ has hitherto been detected only in pieces of branch- 

 ing coral ; I have found it on Sideropora digitata and palmata, 

 and on species of Seriatopora ; Verrill found it on Pocillopora 

 ccBspitosa, in the Sandwich Islands, and D. Graeffe discovered it 

 on two species of Seriatopora. On all these corals the crabs 

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

 branch ; these growths, which are sometimes very broad and 

 massive, and sometimes very slender, grow opposite each other 

 in such a way that the crab settled between them is perfectly 



