Hapalocarcinus, the Gall-forming Crab, etc. 51 
is divided into two chambers, anterior or cardiac and posterior or 
pyloric. The cardiac chamber is a large spherical sac, the cuticle of 
which has been thickened locally to form a series of plates bearing teeth, 
the so-called gastric mill, an apparatus which continues the task, begun 
by the appendages, of breaking up the food into very small particles. 
The cardiac and pyloric cavities are partly separated by a valve which 
leaves only a narrow channel of communication and this, together with 
the anterior part of the pyloric cavity, is occupied by innumerable 
sete springing from the unthickened cuticle and the ossicles. These 
form an effective sieve for preventing the passage of any but the small- 
est food particles into the mesenteron. In the posterior part of the 
pyloric region the chitin of the ventral wall is thickened to form the 
pyloric ampulle, which consist of an elaborate arrangement of parallel 
ridges with rows of sete springing between them, and constitute an- 
other filtering apparatus to which the food current is finally subjected. 
ex.mp.l, 
ain 
a Fic. 9.—Buccal appendages of 
one side. X90. 
First maxilliped, showing epipo- 
dite, ep.; exopodite ex. mp. 1; 
endopodite, end.; basipodite, 
B; and coxopodite C., 
‘i | y S PSST Mire Second maxilla Mz. 2, and scap- 
C. Met hognathite Sc. First maxilla, 
Mz. 1; mandible, Md. 
\\ 
In Hapalocarcinus this arrangement is very much simplified. In the 
cardiac chamber many of the plates have entirely 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 thinner and the teeth they bear, instead of being stout and blunt, 
are long and slender, passing into the sete. Figure11 shows, side by side, 
the zygocardiac ossicles, from Hapalocarcinus and from a very young 
and small Carcinus (not exceeding the first greatly in size). Their dif- 
ferent nature and function will be readily appreciated. The remains 
of the gastric mill of Hapalocarcinus, no longer used in mastication, 
apparently aid the setz of the pyloric valve in sieving the food current. 
At the same time it must be remembered that in other Brachyura the 
ossicles do sometimes give rise to occasional setz; it is, however, usual 
to find them suppressed to form tubercles or fused to form teeth which 
are useful in the comminution of food. It is possible that to a large 
extent the formation of the plates of the gastric mill is due to the fusion 
of individual sete, so that Hapalocarcinus has passed through a retro- 
gressive process of evolution. 
