20 OSBORN. [Vol. XVIII. 



Apohlema, but without finding any. There are distinct longi- 

 tudinal fibers in the collecting vessel wall (Fig. 22), though they 

 are not very numerous. I have not seen any circular fibers, and 

 believe that they are totally .wanting. 



The recurrent vessel is directly continuous with the collecting 

 vessel, and of the same diameter, but it differs in being supplied 

 with frequent strong flagella, as are all the rest of the vessels with 

 the exception only of the capillaries next the flame-cells. The 

 recurrent vessel does not receive any capillaries, but these enter its 

 two branches, which form near the level of the ventral sucker (Fig. 

 23), and one of which runs forward and the other backward. All 

 of the smaller draining vessels are tributaries of these forks. All 

 these parts are supplied with numerous comparatively large flagella 

 (Figs. 23, 24, 25, 26) located close to each other. The flagella are 

 not visible in the freshly mounted animal, owing to their great 

 speed, but later, through fatigue perhaps, they slow down and 

 then become readily visible. Each one has a thickened base and 

 tapers to a point at a distance from the base considerably greater 

 than the diameter of the vessel. In motion they have the appear- 

 ance of a rapidly turning screw, and must produce a strong current 

 in the direction of the collecting vessel as all are located so that 

 the free end is in that direction. I have not as yet succeeded in 

 recognizing the cell-structure of the flagellum or of the cells in 

 the walls of these vessels. 



As already noted, the recurrent vessel here is not like it is in 

 Aspidogaster, where it arises in the front end of the " foot " and 

 first runs backward and then forward to the neck, when it returns 

 and takes a posterior course, branching once in the anterior part of 

 the body and then, near the middle, dividing into three branches 

 instead of two as in Cotylaspis. Further, the recurrent vessel is 

 smaller than the collecting vessel. In Stichocotylc (Nickerson 

 '95) the recurrent vessels are still different; a small recur- 

 rent vessel arises (called by Nickerson " collecting tubule ") near 

 the end of the large " excretory vesicle," which is the homologue of 

 the collecting vessel of Cotylaspis; near the pharynx this vessel 

 is supplied with flagella as in Cotylaspis, but it is represented as 

 branching in a number of different places, and at these points is 

 somewhat swollen. 



