376 FRESH-WATER BIOLOGY 

 13 (27) Posterior disc with six suckers 



14 (26) Posterior disc terminal; suckers large. 



Polystoma Zeder 1800 . . 15 



Six suckers in a circle or in two rows somewhat separated in the median line. In the center 

 of each sucker a small hook, and others on anterior and posterior margins of shield; between 

 posterior acetabula two large hooks. Vagina double, one pore on each side near the ante- 

 rior end. Eggs without polar filament. Genital atrium with circle of hooks. 



Several species in reptiles and amphibians. Not common but widely distributed. P. 

 integerrimwn Zeder, type of the genus, is not reported from North America. American species 

 worked out by Stunkard. 



All North American forms fall in the section of the genus characterized by the presence of 

 a short uterus containing a single egg; to these forms a new subgeneric name should be given. 



Polystoma (Polystomoides) Ward. 



15 (23) Great hooks present on caudal disc and well developed 16 



16 (22) Genital hooks of equal length 17 



17 (18, 21) Not more than 16 genital hooks. 



P. (Polystomoides) hassalli Goto 1899. 



Length i .3 to 2 mm. ; width 0.4 to 0.65 mm. 

 Caudal suckers 0.12 to 0.16 mm. in diameter. 

 Caudal disc with 18 hooks, the largest 0.125 

 mm. and the smallest 0.033 mm. long. Cir- 

 rus hooks 0.028 mm. long with a winglike 

 process at the middle. Uterus contains only 

 a single large egg measuring o.n by 0.25 mm. 

 to o.i 8 by 34 mm. 



Urinary bladder of Cinosternum pennsylvan- 

 icum, Aromochelys carinatus, A. odoratus, Chel- 



FIG. 653. Polystoma hassalli. Ventral view. X 18. ^ serpentina; Maryland, North Carolina, 

 (After Stunkard.) Texas, Iowa. 



18 (17, 21) Genital hooks 32 19 



19 (20) Acetabula large, adjacent, not contiguous; pharynx smaller than 

 oral sucker. . . P. (Polystomoides) coronatum Leidy 1888. 



Body 3 . 1 5 by 0.83 mm. Caudal suckers 

 0.37 mm. in diameter. Caudal disc with 

 one pair of great hooks, 0.132 mm. long, 

 one pair of intermediate hooks, 0.051 mm. 

 long, and small hooks, 0.02 mm. long. 



From the common food terrapin (Leidy). 



FIG. 654. Polystoma coronatum. Ventral view. 

 X 9- (After Stunkard.) 



