Mr Potts, The gall-forming Crab, Hapalocarcinus. 461 
between the state of development and the size of the crab it 
contains. In all cases the gall appears to be formed and in- 
habited by a solitary female individual. Those in the younger 
“open” galls, where the two constituent branches have not yet 
approached each other and fused, are immature: those in the 
older “closed” galls are mature and often the abdominal appendages 
are laden with developing eggs. 
The young female crab probably commences its sedentary life 
by settling down in the notch at the apex of a recently divided 
branchlet. She is at this period a small flat creature, little more 
than a millimetre in carapace length. Her position at the growing 
point allows her to control the future development of the branch. 
The initial modification of growth is probably due to the mere 
mechanical effect of the continued presence of the animal. The 
two branches, instead of remaining cylindrical, broaden out. They 
then approximate above and laterally, thus partially enclosing a 
chamber which is large enough to contain the crab with comfort. 
In the diagrams which illustrate this paper the chamber is referred 
toas 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 time 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 the 
ovary begins to grow rapidly, causing so great a development of 
the abdomen that more spacious accommodation is quickly needed. 
This is provided by the new chamber (B) of the gall. 
It follows then, that since the crab only enters into occupation 
on completion of the work, she must influence the growth of the 
branches in a manner more indirect than, though quite as effective 
as, at first. It is rather difficult to explain the precise influences 
which give the gall its characteristic and constant shape, but it 
can be stated that if the respiratory current of the crab is not the 
sole external factor which is responsible it is at any rate the most 
important. By means of powdered carmine spilt in a small 
quantity of sea water containing a crab I was able to assure myself 
that, as a general rule, the water needed for respiration is sucked 
into the branchial chamber behind and expired upwards and out- 
wards, as usually happens in the Brachyura. The form of the 
initial cavity A, a narrow crevice, hardly allows the animal to 
move except in one plane. Thus the upward and outward current 
has sufficient fixity of direction and constancy of power to secure 
a definite result. This current either flows straight upwards or 
impinges on the lip of the chamber A. It the latter case it 
would be deflected obliquely toward the opposite lip, tending to 
