Hapalocarcinus, the Gall-forming Crab, etc. 



53 



"nannoplankton," which embraces all those animals and plants less 

 than 3 to 4ju in diameter. 



The currents of water drawn into the gall are of course caused by the 

 action of the scaphognathite, for there does not appear to be any other 

 mechanism of the calibre necessary for creating currents. The water 

 used in respiration is, as is usual, drawn into the branchial cavity pos- 

 teriorly and breathed out anteriorly. But the organisms contained in 

 it, if not entirely prevented from entering the branchial cavity by the 

 filter of setae guarding the entrance, will at any rate partly be caught 

 in eddies circulating under the ventral surface and probably in minor 

 currents caused by the motion of the buccal appendages. The buccal 

 area is so widened and the appendages so modified and dislocated that 

 a maximum of water can circulate in the neighbourhood of the mouth. 



11 



FIG. 11. Zygocardiac ossicles of: A, Carcinus; B, Hapalocarcinus. 

 FIG. 12. Urocardiac ossicle of Hapalocarcinus. X220. 



X220. 



I suppose that the plankton is separated from the respiratory current 

 by movements of the nets of setae on the maxillipeds, resembling the 

 sweeping action of the legs of Cirripedes. The smallest members of the 

 plankton only, in view of the minute size of the apparatus, would be 

 entrapped. They would then be sucked into the stomach by pulsating 

 movements such as Mocquard has described in other Decapoda. He 

 has shown that the stomach functions in the manner of a pump by 

 the alternate action of the dilator and constrictor muscles. By the 

 expansion of the cardiac sac the partially chewed food is sucked into the 

 stomach and on its constriction the fluid taken in is expelled, while the 

 solids are retained, largely owing to the action of the subcesophageal 

 valve. It will be observed that notwithstanding the simplification of 

 structure in the stomach of Hapalocarcinus there is still left the combi- 



