46 ILLINOIS BIOLOGICAL MONOGRAPHS [326 



KEY TO THE SPECIES OP THE GENUS POLYSTOMA 



Uterus long, contains many eggs 2 



Great hooks present on the caudal disc _ 3 



Ceca branching _ P. integerrimum 



Ceca not branching P. bulliense 



Great hooks not present on caudal disc P. alluaudi 



Uterus short, contains a single egg _ 7 



Great hooks present on caudal disc _ 8 



Genital hooks of equal length _ _ 9 



Not more than sixteen genital hooks „ „ 10 



Genital hooks eight in number; 



ectoparasitic form P. integerrimum 



Genital hooks sixteen in number P. hassalli 



Genital hooks more than sixteen in number 13 



Genital hooks thirty-two in number 14 



Caudal suckers large, adjacent but not contiguous, 



pharynx, smaller than anterior sucker P. coronation 



Caudal suckers small, widely separated, pharynx equal in 



size to anterior sucker P. microcotyle 



Genital hooks more than thirty-two in number 17 



Testis simple „ 18 



Caudal suckers large, overlap P. megacotyle 



Caudal suckers small, separated P. ocellatum 



Testis branched - P. kachugae 



Genital hooks unequal in length P. oblong um 



Great hooks of caudal disc reduced or absent 23 



Genital hooks sixteen in number P. orbicular e 



Genital hooks thirty-two in number P. opacum 



