Hapalocarcinus, the Gall-forming Crab, etc. 



41 



branch of which it forms part. A perfect forest of twigs covers the 

 surface of galls of some standing and occasionally examples are to be 

 observed in which the gall is the foundation of a complex branching 

 system. In these cases the galls can evidently lay claim to a respect- 

 able antiquity. As long as the crab lives the cavity remains in con- 

 nection with the exterior by means of the circle of apertures. Sometimes 

 a gall is seen where the crab is dead and all apertures have closed up. 



THE GALLS IN SERIATOPORA HYSTRIX. 



This coral grows only in the sheltered waters of the inner reef, though 

 very widely distributed there, and there is little modification of the 

 manner of branching. The colony consists of slender cylindrical 

 branches of approximately equal diameter, diminishing slowly towards 



FIG. 3. Gall-formation in Seriatopora hystrix. 



A. Fragment of normal colony, showing dichotomous branching and fusion of adjacent branches. 



B, C, D. Views of a single gall formed at the junction of three branches, 1, 2, 3. 



B. One of its component laminae formed by growths of crenenchyme from branches 2, 3; the 



outlines of these branches are still preserved ; the black spots show the thecse of the 

 colony regularly arranged on the original branches; not so much so on the new growths. 



C. Side view showing processes from branches 1, 2 fusing together. 



D. Similar view of other side, showing junction of processes from branches 2, 3. 



E. Another gall, showing how each lamina is made up of a number of small branches fused 



together. (From a specimen in the Cambridge University Museum of Zoology.) 

 Natural size. 



the sharp apex. It has then a very spiky appearance. The branching 

 is regularly dichotomous and where adjacent branches come into con- 

 tact they fuse, as is seen in the accompanying figures. Where branch- 

 ing takes place at the apex the space between the branchlets is entirely 

 inadequate to shelter even so small a creature as the young Hapalo- 

 carcinus. For this reason the galls are found not at the surface of the 

 colony, as in Pocillopora, but between more basally situated branches 

 where a broader lodgement is offered. 



When the crab is established a very marked interference with the 

 growth of the coral takes place. The normal growth is greatly stimu- 

 lated, a much more considerable addition to the ccenenchyme appearing 



