317] NORTH AMERICAN POLYSTOMIDAE—STUNKARD 37 



POLYSTOMA MEGACOTYLE Stunkard 1916 

 [Figures 22 to 26] 



The material of this species consists of three specimens from the 

 mouth of Chrysemys marginata from Creston, Iowa. One worm was 

 cut into cross sections and the other two mounted as stained toto 

 preparations. 



These worms (Fig. 22) have an elongate ovoid shape. Widest in 

 the region just anterior to the caudal disc, they gradually become nar- 

 rower anteriorly, and posteriorly they taper rapidly to a caudal tip 

 which is set in thfe antero-central part of the caudal disc. The worms 

 are 2.5 to 2.7 mm. long and 0.71 to 0.78 mm. in width. The caudal disc 

 is cordiform and the suckers are so large that they slightly overlap each 

 other. The suckers are arranged in about four-fifths of a circle around 

 the lateral and caudal margins of the disc. Measurements thru the disc 

 from side to side at the level of the cephalic suckers are from 1 to 1.4 

 mm., thru the middle pair 1.2 to 1.8 mm., and thru the caudal suckers 

 0.68 to 0.7 mm. The disc bears the characteristic armature of hooks. 

 Across the anterior margin there are three larval booklets in one speci- 

 men and four in the other, but their arrangement is not regular or 

 definite and their position would indicate that they do not function in 

 attachment. In the specimen reproduced in Figure 22 the two hooks 

 of the right side have their points almost together and their bases apart. 

 In the bases of the suckers there are small larval booklets, and one pair 

 similar in size and shape between the two caudal suckers. Also between 

 the posterior suckers (Fig. 41) there is the pair of great hooks and a 

 pair of hooks the same shape as the great hooks and intermediate in size 

 between the great and larval hooks. The hooks measure in length : lar- 

 val 0.017 mm., great hooks 0.116 mm., and the pair intermediate in size 

 0.058 mm. 



The cuticular covering of the body is approximately 5/a in thick- 

 ness on the dorsal and 3 to 4/a in thickness on the ventral surface. It 

 is turned in at the external openings and lines the digestive tract to the 

 bifurcation. 



The anterior sucker is set off from the remainder of the body by 

 a slight constriction. It is oval, its longest axis crosswise of the body, 

 somewhat flattened posteriorly, and measures 0.28 mm. in length by 

 0.35 to 0.42 mm. in width. It is followed by the pharynx (Fig. 25) 

 which is 0.35 to 0.38 mm. long, 0.38 to 0.44 mm. broad, and in the sec- 

 tioned worm 0.34 thick. No esophagus was observed; the ceca meet 

 anteriorly in a wide curve and extend almost to the posterior end of the 

 body. They are 0.06 to 0.11 mm. in diameter, and have an epithelial 



