CRUSTACEANS IN CILETOPTERUS Tl IU.S. 



107 



stant bolh as to force and direction. In Polyonyx, however, 

 such currents are strong and well adapted to life in tubes. When 

 an individual is placed in a dish of water, a current will soon be 

 seen to issue from beneath the anterior end. Sometimes it 

 passes straight forward, but more often moves to the right or 

 left, as it would have to do if the crustacean were in a tube. 



FIG. 3. Cleaning leg of Polyonyx. a, entire leg with details of pectinate 



setae; b, chelate tip of leg. 



The direction of the respiratory current in a particular individual 

 may change frequently in a dish, but in a tube it is usually 

 constant for hours at a time. Furthermore, several individuals 

 in a tube together will adopt the same direction so that the water 

 passes in one end of the tube and out the other. Polyonyx's 

 antennules assume a peculiar position in relation to the respira- 

 tory current. Both are bent against the current and take such 

 a position that the smooth ramus (Fig. 50) meets it first; and if 

 the current is changed, the position of the antennules is always 



