FEEDING, LOCOMOTION AND RESPIRATION T>5 



small openings near the bases of the antennae. The position of this 

 outflow is helpful in removing the products of the excretory organ, 

 which opens on the base of the antenna. 



The surfaces of the gills are kept clean by the action of long 

 processes (epipodites) extending backwards from the bases of the 

 maxillipeds. These epipodites sweep over the gills and dislodge any 

 particles which may have entered with the respiratory current. The 

 current can also be reversed, so that water flows into the gill 



Fig. 24. Polybius henslowi (Decapoda, Brachyura), a 

 swimming crab; note the large paddles at the ends of the 

 last pair of legs. Although this crab is about the same 

 size as the shore crab, Carcinus, its exoskeleton is much 

 thinner and lighter. Actual width of carapace about 

 5 cm - 



chamber through the usual exit. This is a useful additional 

 mechanism for clearing the gill chamber of foreign particles. 



Some crabs make regular use of the reversed current. The masked 

 crab, Cory st es cassivelaunus, lives in fine sand, and buries itself 

 below the surface. The antennae are long, and each bears two rows 

 of setae which overlap those of the other antenna and form a tube 

 leading to the sand surface. By using the reverse respiratory current 

 water is drawn down through this tube and is passed out through 

 the openings at the bases of the legs. A similar modification is 



