374 FRESH- WATER BIOLOGY 



Even concerning the forms listed it must be confessed that our 

 knowledge is very imperfect. 



In preparing the key I have followed the plan so admirably 

 formulated by Looss and worked out in various groups by Braun, 

 Liihe, and Odhner. The data on larval forms (Cercariae) are 

 adapted from Cort and Faust. 



KEY TO NORTH AMERICAN FRESH-WATER TREMATODA 



1(169) Adult forms; sex organs developed and functioning .-. . .' 



2 (28) Posterior organs of attachment powerfully developed; those at ante- 



rior end absent or if present poorly developed and paired. 

 Chitinous hooks and anchors almost always present. 



Subclass Monogenea . . 3 



Excretory pores anterior, double, dorsal; uterus short usually containing only a single egg. 

 Development simple, direct. Most forms are ectpparasitic on body surface or gills. In 

 fresh-water hosts found in urinary bladder (Amphibia) or respiratory passages (turtles). 



3 (8) Posterior organ single. Vagina unpaired. No genito-intestinal canal. 



Order Monopisthocotylea Odhner . . 4 



4 (5) Two suckers at anterior end, entirely independent of the oral cavity. 



A single large posterior sucker. 



Family TRISTOMIDAE van Beneden 1858. 



Monogenetic, ectoparasitic trematodes with a single large round terminal sucker, often 

 armed with hooks, and with two smaller yet conspicuous lateral suckers at the anterior end. 

 Mouth ventral just behind anterior suckers. Many forms parasitic on gills of marine fishes; 

 a few reach fresh water through the movements of migratory fish. 



Only species reported from North America. 



Nitzschia sturionis (Abildgaard) 1794. 



Reported by Lin ton from gills of sturgeon (Acipenser sturio) at Woods Hole. May be 

 carried at times into fresh water. 



5 (4) Anterior end expanded, bearing special structures of some sort and yet 



never true suckers alone. 

 Family GYRODACTYLIDAE van Beneden and Hesse 1863 . . 6 



Small, slender, elongate trematodes with anterior end variably provided with specialized 

 structures, only rarely true suckers and then associated with other special organs. Posterior 

 disc without suckers, usually with two or four huge hooks in the center and a considerable 

 number of small marginal booklets. 



On the skin and gills of fishes. 



The genera reported from fresh water all fall in the section of the family in which the an- 

 terior end is provided with two or four retractile cephalic tips in which open ducts of numer- 

 ous dermal glands. 



6 (7) Posterior disc with two large central hooks. No eyes. 



Gyrodactylus von Nordmann 1832. 



Anterior end provided with two lateral contractile lappets. Large central hooks of pos- 

 terior disc turned ventrad, shaped like fish hooks and bound together at the roots by a special 

 clamp piece. Marginal hooks sixteen, simple. Viviparous. 



On skin and gills of many fresh-water fish, especially Gyprinidae. At times numerous enough 

 to destroy the external dermal layer and leave the fin rays naked. May cause death of host. 



Reported only twice in North America; from young lake trout in Maine and small-mouthed 

 black bass, Ontario, Canada. Species uncertain. Cause of serious epidemic among young 

 fish at hatchery (Craig Pond); also on wild fish in same stream. 



