464 Mr Potts, The gall-forming Crab, Hapalocarcinus. i 
the latter is known. This crab inhabits deep pits within Astraeid 
colonies, each pit in its origin a single theca. The pit contains, 
as a rule, a male as well as a female, and though neither so rare 
nor so roving as in Hapalocarcinus he is also far smaller than the 
female (one quarter her size). The Hapalocarcinidae (a family 
formed by Calman to include these two genera) thus furnishes by 
far the most marked cases of sexual dimorphism to be found in 
the Decapoda. 
Certain points of structure may perhaps be mentioned now as) 
shedding light on the biology of Hapalocarcinus. Calman has: 
described the wide buccal area and pointed out that there is a/| 
certain amount of reduction in the third maxilliped. This is very | 
much accentuated in the maxillae and mandibles which Calman‘ 
was not able to examine owing to paucity of material. Hach | 
member of the two pairs of maxillae is reduced to a single 
elongated plate. The mandible is a well developed triangular 
piece of chitin but without the denticulate biting border of the | 
typical Decapod mandible. Especially noticeable too is the absence 
of a mandibular palp. It is plain that none of these appendages: 
are used for mastication. 
A similar modification is to be noticed in the stomach. In all 
the higher Crustacea the chitinous wall of the cardiac chamber is , 
thickened locally to form a system of plates bearing teeth, the 
so called gastric mill, which continue the task of breaking up the. 
food into very small particles. ‘The pyloric chamber is occupied 
by a very efficient filtering mechanism composed of interlacing | 
setae which only allows food in an easily absorbed condition to 
pass into the midgut. In Hapalocarcinus this arrangement is 
very much simplified. Many of the plates have disappeared and | 
while the more important constituents of the gastric mill, the uro- | 
cardiac and zygocardiac ossicles, are still present, they are much 
weaker and the teeth they bear instead of being stout and blunt 
are long and slender, passing into setae. Their function is not 
mastication but they apparently aid the setae of the pyloric valve ‘ 
in sieving the food current. In accordance with this forward. 
shifting of the sieving mechanism the structure of the pyloric | 
chamber is simpler and the pyloric ampullae which are so promi- - 
nent a feature in other Decapoda are entirely unrepresented. 
This modification, both of the mouth appendages and of the | 
stomach, is far greater than any which occurs in other Brachyura 
and we must look to a very distinct cause for the explanation, 
This cause is, without doubt, the change in the feeding habits of - 
the animal caused by its voluntary imprisonment in an almost | 
totally closed space. The holes which allow entrance to the 
“closed” gall are exceedingly small in the skeleton and must be 
smaller still if we allow for the coating of living coenosarc. It has 
f 
