378 



FRESH-WATER BIOLOGY 



15 (24) Genital hooks 32 in number. 



P. {Polystomoides) opacum Stunkard 1916. 



Length 3 to 4 mm., width 0.8 to i mm. Caudal suckers 0.4 mm. in 

 diameter. On caudal disc many small hooklets 7 to 9 /x long, and one 

 larger pair, 75 /x long; no great hooks present. Genital coronet of 32 

 i.3>c>l) equal hooks. Egg oval, 0.25 by 0.2 mm. 



Vitellaria of large compact follicles under dorsal surface from pharynx 

 to caudal disc except over ovarian complex; so extensively developed as 

 to obscure internal organs and render body opaque. 



In esophagus of Trionyx ferox and of Malacoclemmys lesueurii; 

 Texas. 



Fig. 658. Polystoma opacum. 



Extended. 

 Stunkard.) 



Ventral view. X 7- (After 



26 (14) Posterior disc overhung by a flap bearing four hooks. 



Diplobothrium F. S. Leuckart 1842. 



Elongate, ectoparasitic trematodes with slender posterior end. Six short stalked suckers 

 arranged in two longitudinal rows and armed with chitinous hooks, stand just anterior to 

 slender caudal tip which carries two hooks on each side. 



One species {D. annatum) reported on the gills of the lake sturgeon (Acipenser rubicundus) 

 from St. Lawrence River. 



27(13) Posterior disc with two suckers Sphyranura Wright 18 jg. 



Caudal lamina considerably wider than slender body, with two im- 

 mersed suckers, two large hooks behind them, and sixteen small hooks 

 arranged seven along each side of the lamina and one in each sucker. 

 Two contractile bladders anteriorly, each with a dorsal pore. No lateral 

 vaginae. Oviparous. 



Only species known. 



Sphyranura osleri R. R. Wright 1879. 



On the skin of Nedurus lateralis in the Great Lakes region. Corre- 

 sponds to larval stage of Polystoma in having only two terminal 

 suckers. 



Fig. 659. Sphyranura osleri. 



Ventral surface. 

 MacCallum.) 



X 20. (After Wright and 



