311] NORTH AMERICAN P0LYST0M1DAE—STUNKARD 31 



line, just posterior to the bifurcation of the digestive tract. It opens 

 from a common genital sinus (Figs. 13, 26) into which the uterus dis- 

 charges and thru which the cirrus is extruded. The opening from the 

 uterus into the genital sinus is posterior and ventral, while the cirrus sac 

 opens into the dorsal part of the atrium. 



When the two specimens of P. opacum from Trionyx ferox were 

 placed in a watch glass, they soon came in contact and immediately 

 started copulation, the cirrus of each worm was inserted in the right 

 vagina of the other, and the two worms attached to each other, both with 

 the anterior suckers and those of the caudal disc that could be brought in 

 position for adhesion. Attempts to separate the worms failed, so an 

 effort was made to fix them in the copulating condition, but they separated 

 on the application of the killing fluid. This explains the statement of 

 Johnston : ' ' On one side only, in the specimens sectioned, was the 

 vaginal tube filled with sperms; that on the other side was empty." 

 Benham (1901) and Mac Callum (1913) state that copulation in poly- 

 stomes has been observed only by Zeller. 



POLYSTOMA ORBICULARE Stunkard 1916 

 [Figures 1 to 14] 



The material of this species consists of six specimens from the 

 urinary bladder of Pseudemys scripta from Raleigh, North Carolina, 

 one specimen from the urinary bladder of Chrysemys marginata from 

 Chicago, Illinois, and two specimens from the urinary bladder of Chry- 

 semys marginata from Creston, Iowa. 



The body is an elongate oval, slightly more pointed anteriorly than 

 posteriorly, and in two of the specimens with slight indentations of the 

 body walls at the vaginae and at the posterior margin of the anterior 

 sucker. These worms (Fig. 1) varied in length from 2.7 to 3.75 mm. 

 and in width from 0.9 to 1.2 mm. The caudal disc is circular, 0.8 to 

 1.07 mm. in width, and bears the six suckers arranged symmetrically 

 in a circle. The suckers are approximately 0.3 mm. in diameter, and 

 are separated by regular equal intervals. No hooks could be found on 

 the caudal disc with the exception of the single minute larval hooklet 

 in the base of each sucker. These are 0.016 mm. in length and could be 

 seen only under favorable conditions. 



The anterior sucker (Fig. 6) is 0.25 to 0.27 mm. in length and 

 0.37 to 0.42 mm. in width. It opens into the pharynx, a spherical struc- 

 ture 0.24 to 0.28 mm. in diameter. There is a short esophagus visible 

 in sagittal sections altho it is not distinguishable in toto preparations. 

 The ceca meet anteriorly in a wide curve and extend as simple tubes 



