158 THE STORY OF LIFE IN THE SEAS. 



On the shores of the Philippine Islands and 

 in other parts of the Pacific Ocean there is a very 

 common coral named Seriatopora. It is com- 

 posed of numerous delicate branches, terminat- 

 ing in fine pointed extremities, forming hemi- 

 spherical shrub-like masses, six or eight inches 

 in diameter. Semper noticed that on some of 

 the branches of these Corals there were little 

 heart-shaped swellings, 

 which had the appear- 

 ance of malformations or 

 structures corresponding 

 to the galls on the leaves 

 and branches of trees. 

 Each of these swellings 

 contained a cavity, com- 

 municating with the ex- 

 terior by two minute 

 holes, in which there 

 was imprisoned a small 

 Crab. 



By the examination 

 of a large number of 



FIG. 40. A Crab-gall on a branch ^^im^c, Q PTYin p r PQTYIP 



of a Seriatopora. specimens, fcempei 



to the conclusion that 



the history of these structures was somewhat as 

 follows. The young Crab, when it settled down 

 on the branch, produced an irritation which in 

 some way caused a gall-like growth of the tissues of 

 the Coral. This growth continued until it formed 

 at first a case or sheath for the protection of the 

 Crab, and eventually, as the Crab increased in size, 

 a cage from which it could not do more than 

 protrude its tentacles and claws when feeding. 



