Hapalocarcinus, the Gall-forming Crab, etc. 39 



an equal distance apart. The initial tendency of the branch to broaden 

 out is encouraged, but at the same time the tendency to dichotomous 

 branching is suppressed. There are thus formed two broad expansions, 

 which approximate above and laterally, thus partially enclosing a 

 chamber large enough to contain the crab with comfort. In the 

 diagrams which illustrate this paper this chamber is referred to as A. 



In the second stage of construction of the gall a much larger upper 

 chamber (B) is formed by the continued growth of the two branches. 

 During nearly the whole of this time, however, the crab remains an 

 inhabitant of A. It is not until the two walls of the upper chamber 

 have almost met that she finds her earlier quarters too restricted and 

 transfers herself to the upper chamber at the time when it is ready for 

 occupation. Just before fertilisation, which occurs shortly after this 

 removal, the ovary begins to grow rapidly, causing so great a develop- 

 ment of the abdomen that more spacious accommodation is quickly 

 needed and provided for in the new chamber. 



It is rather difficult to explain the precise influences which give the 

 gall its characteristic and constant shape, but it can be stated with 

 certainty that if the respiratory current of the crab is not the sole exter- 

 nal factor which is responsible, it is at least the most important. The 

 initial modification of growth may be due in part to the mere mechan- 

 ical effect of the continued presence of the crab, but I think even here 

 the respiratory current is more effective. By means of powdered 

 carmine spilt in a small quantity of sea-water containing a crab I was 

 able to convince myself that, as a general rule, the water needed for 

 respiration is sucked into the branchial chamber behind and expired 

 upwards and outwards, as usually happens in the Brachyura. Even 

 at the very first, we must suppose that the crab never forsakes its 

 position in the saddle between the two branchlets. It may move 

 laterally, but always in the same plane. Later, when the initial cavity 

 A has taken shape as a narrow crevice, movement except in the one 

 plane becomes an impossibility. Thus the original expiratory current 

 always flowing upwards and outwards has sufficient fixity of direction 

 and constancy of power to secure a definite result. 



In the first place, then, the effect of a vertical current of water flowing 

 between the two branchlets is that the further growth of these tends to 

 follow the borders of the stream, so that they approximate and do not 

 diverge. With regard, however, to the formation of the larger chamber 

 #, the crab enters into occupation only at the completion of the work. 

 It follows, then, that here she must influence the growth of the branches 

 in a more indirect, though quite effective manner. The expiratory 

 current either flows straight upward through the entrance to chamber A 

 or impinges on the lip. In the first case its velocity would be checked 

 and it would tend to disperse on entering the larger body of water. 

 Or if it strikes the lip of the chamber it would be deflected obliquely 

 toward the opposite side. In either case the effect will be to secure a 



