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 

 setae 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 ampullce, which consist of an elaborate arrangement of parallel 

 ridges with rows of setse springing between them, and constitute an- 

 other filtering apparatus to which the food current is finally subjected. 



ej.mp. 



FIG. 9. Buccal appendages of 

 one side. X90. 



First maxilliped, showing epipo- 

 dite, ep.; exopodite ex. mp. 1; 

 endopodite, end.; basipodite, 

 B; and coxopodite C., 



Second maxilla MX. 2, and scap- 

 hognathite Sc. First maxilla, 

 MX. 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 setse. Figure 1 1 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 setse 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 setse; 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 setse, so that Hapalocarcinus has passed through a retro- 

 gressive process of evolution. 



