319] NORTH AMERICAN POLYSTOMIDAE-STUNKARD 39 



The descending and ascending ducts are 6 to 11/t in diameter ; when 

 empty their walls collapse. 



P. megacotyle differs from all known American forms in the large 

 number of hooks present in the genital coronet, and in this character 

 agrees only with P. ocellatum. The species differs from P. ocellatum, 

 however, in the difference in size of the anterior sucker and pharynx 

 as well as in the size of the caudal suckers. P. megacotyle differs from 

 P. microcotyle in the number of genital hooks and in the size of the 

 posterior suckers. P. megacotyle has a larger pharynx, larger caudal 

 suckers, and a larger number of gental hooks than P. coronatum. 



I 

 POLYSTOMA MICROCOTYLE Stunkard 1916 



[Figures 28 and 29] 



This species is described from a single specimen from the mouth 

 of Chrysemys marginata from Creston, Iowa. The worm was stained 

 and mounted in toto (Fig. 28). 



It is 3 mm. long, and 0.78 mm. in width. The caudal disc is cordi- 

 form, 1 mm. in width at the level of the anterior suckers, 1.07 mm. 

 thru the middle pair, and 0.74 mm. thru the caudal pair of suckers. 

 Each sucker is 0.28 mm. in diameter and with the exception of the 

 longer distance between the anterior suckers, they are separated by 

 almost regular equal distances. The distance between the anterior 

 suckers is about four times as great as that between the posterior pair. 

 Four larval booklets are present between the two anterior suckers, three 

 in a row but with their hooks pointing in different directions, and the 

 fourth some distance posterior to the others (Fig. 29). Between the 

 posterior suckers there are three pairs of hooks : the pair of great hooks, 

 one pair of larval hooks, and a third pair intermediate in size between 

 the great and larval hooks. The hooks of this third pair are the same 

 shape as the great hooks. The larval hooks are 0.017 mm. long, the 

 great hooks are 0.116 mm. long, and the pair intermediate in size are 

 0.061 mm. long. 



In this specimen as the suckers are small the musculature of the 

 caudal disc shows very plainly (Fig. 29). Muscle strands from the 

 ventral side of the body and others from the body wall pass to the bases 

 of each of the suckers. Others pass to the outside of the different suck- 

 ers and are inserted on the distal and intermediate zones of the suckers, 

 serving as retractors in the operation of the organs. Many break up 

 into smaller fibers and can not be traced. From the base of each sucker 

 the muscles spread out in a fan shaped manner and fibers can be traced 

 not only to the large strands from the body wall but also small fibers 



