Hapalocarcinus, the Gall-forming Crab, etc. 53 
“nannoplankton,”’ which embraces all those animals and plants less 
than 3 to 4u 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 setee 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. 
ener eee 
LP 
Fic. 11.—Zygocardiac ossicles of: A, Carcinus; B, Hapalocarcinus. X220. 
Fig. 12.—Urocardiac ossicle of Hapalocarcinus. X220. 
I suppose that the plankton is separated from the respiratory current 
by movements of the nets of sete 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- 
