io6 



A. S. PEARSE. 



backward. The largest and strongest leg, the fourth, served as 

 a sort of hook to grasp any inequality in the wall of the tube, 

 and the smaller fifth leg was often used in much the same way. 

 In a horizontal tube Pinnixa moved sideways or stood in a 

 position like that when on a flat surface; Polyonyx usually rested 

 on the fringe of bristles along the chelipeds with its head down. 

 Pinnixa' s most effective locomotor organs are the fourth walking 

 legs; Polyonyx uses the chelipeds most. 



Pinnixa is an expert burrower, but Polyonyx has little ability 

 in that line. When placed in a bowl containing sand and sea 

 water a Pinnixa would scratch for a moment with all the .walking 



FIG. 2. Polyonyx and Pinnixa in Syracuse watch glasses. X 4/7. a, Poly- 

 onyx- male at left, female at right, b, Pinnixa; the lower individual is covered 

 by the bryozoan Hippuraria elongala Osborn. 



legs except the chelipeds, then stop moving the legs on one side 

 and quickly burrow sideways by scraping the sand away with 

 those of the other. Usually one would bury itself completely 

 in a minute or two. If Polyonyx was placed in the same bowl it 

 did not burrow, in fact was never seen to burrow, though occa- 

 sionally an individual would wiggle itself down into the sand a 

 little. 



Respiration is a matter of importance to an^ animal that 

 lives in a worm tube imbedded in a muddy flat. Yet Pinnixa 

 apparently takes no special precautions to secure well-aerated 

 water and is able to endure a much greater degree of foulness 

 than Polyonyx. Its respiratory currents are feeble and incon- 



