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 
B, 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 
