THE PROCESS OF GROWTH. 223 



cups, but much deeper in. From the margin of the crab's 

 dwelling, properly so called, there is a funnel that widens to 

 the top, and of which the margin, as is shown in the cut (fig. 

 68), is gradually merged in the uppei' prominences of the coral. 

 The crab living in the funnel thus formed was carefully 

 observed by me during a long period of its life, and I was 

 enabled to see that it protruded itself far enough out of its hole 

 to be able to reach with its outstretched fore-claws almost 

 to the highest portion of the funnel. 



The whole conditions here described '"^ allow of no other ex- 

 planation than the following : At first the crab and the coral 

 grow at an equal rate ; for, if the coral grew more rapidly than 

 the crab, an inverted funnel or hollow cone would be formed 

 over the crab, while, if the crab grew the faster, the margin 

 of its cave-dwelling, so long as it was small, could not be exactly 

 on a level with the margin of the contiguous polyp-cups. But 

 when the crab has reached its full length, about seven milli- 

 metres, the polyps outgrow its funnel-shaped dwelling, and 

 would no doubt soon wholly overgrow it, if it were not that 

 they find a certain resistance in the current set up by the crab 

 for breathing and in the movements of the ci'eature ; and this 

 resistance is sufficient to compel the growth of the coral in a 

 particular and determined direction. The two powers in 

 opposition thus reach an equilibrium, and it is their reciprocal 

 action which gives the funnel its characteristic form. 



Here too, as in the former instance, the individual polyps 

 plainly show the effects of the current. While in general the 

 cups are perpendicular to the surface of the coral, in most of 

 those which grow within the funnel this is not the case ; they 

 have an oblique direction upwards, and are most oblique where 

 the strength of the current is greatest, i.e. at the narrow 

 bottom part of the funnel. 



"We see thus, in these two examples, that the same force — 

 namely, the respiratory current caused by a crab — afiects the in- 

 dividual polyps in the same way, forcing them to grow obliquely ; 

 but at the same time it also produces very ditferent efieots, 

 resulting from the different law of growth of the two forms 

 of coral- stock. Thus galls are produced only on branched 



