4 o8 



FRESH-WATER BIOLOGY 



The common European species (D. dendriticum, the old Distoma lanceolatum) is said by 

 Leidy to be frequent in sheep in several western states, but Stiles and Hassall report it as ap- 

 parently not in North America. I have never seen a specimen collected here. Confusion 

 with Opisthorchis and similar forms is common in earlier records. 



North American genus Halipegus Looss 1899. 



Moderate sized distomes with muscular body, round in cross-section, and powerful suckers. 

 Pharynx large, esophagus short, crura extending to posterior end. Genital pore close to 

 pharynx. No cirrus. Testes lateral, near posterior end, symmetrical. Ovary close behind 

 right testis; vitellaria just behind ovary and composed of group of 4 to 5 large follicles on each 

 side. Uterus in crowded transverse coils, filling almost entire body. Eggs extremely numer- 

 ous, small, with long polar filament. In mouth and pharynx of amphibia. 



North American species. . . . Halipegus occidualis Stafford 1905. 

 In mouth and eustachean tube of Rana catesbiana; Canada, Massachusetts. 



152 (149) Ovary between testes. .... . . , . ... . . . [V * . 153 



153 ( X 56) Ovary median or nearly so, hence directly behind anterior tes- 



tis. .... ..'..'.'. . 154 



154 (!55) Genital pore between acetabulum and pharynx. 



Sphaerostoma Stiles and Hassall 1898. 



Small distomes with actively mobile, powerful anterior region and broad posterior region. 

 Suckers powerful. Pharynx present, esophagus long, crura reach into caudal end. Cirrus 

 sac large, cirrus muscular. Testes separate, anterior one near acetabulum on right, posterior 

 one near caudal tip, median. Ovary intermediate but slightly to left of median line. Vitel- 

 laria extensive, lateral, from pharynx to posterior end. Uterus in few coils between posterior 

 testis and acetabulum. With few, large eggs. 



This genus has not yet been reported from fresh-water fishes in North America. Linton 

 has found it in marine fishes in the Woods Hole region and it is common in Europe in Cypri- 

 nidae and many other fresh-water fishes so that it is very likely to be found on this continent 

 in similar fresh-water hosts. 



J 55 ( X 54) Genital pore some distance behind acetabulum, just anterior to 

 anterior testis Clinostomum Leidy 1856. 



Middle sized distomes with flattened body. 

 Oral sucker small and retracted at times so that 

 the body wall rises around it like a collar. Ace- 

 tabulum near oral sucker, larger, very muscular, 

 with triangular orifice. No pharynx, short eso- 

 phagus and long crura provided with lateral 

 pockets. Cirrus sac present. Vitellaria lateral, 

 strongly developed, confluent behind testes. Uter- 

 us inverted U-shaped, reaching forward nearly to 

 acetabulum, with expansion on distal branch of U. 

 Several species in North America. Not clearly 

 distinguished in records. Adults are parasitic in 

 the pharynx and esophagus of fish-eating birds 

 such as herring gull, various herons, bittern, eagle, 

 stork. Young forms encysted in frogs and fish 

 (minnows, perch, bluegill, bullhead, rock bass, 

 pike, black bass, trout, etc.) . Cort has shown that 

 the young encysted in amphibia are a different 

 species from those in fish. Widely distributed in 

 eastern North America at least. The larval stages 

 are so abundant in some regions that food fish are 

 rendered unfit for use by the middle of June. The 

 cysts are deserted by late fall and the fish are free 

 from infection in winter. 



Distoma oricola Leidy from the mouth of Alli- 

 gator mississippiensis is undoubtedly a related 

 form as Pratt surmised. It falls in this family 



FIG. 708. Clinostomum but too little is known of its structure to justify tomum 'marginatum. 

 marginatum. Larval stage assigning it to a definite genus. Young adult from 



from perch. X 19. (After heron. X 19. (After 



Cort.) Cort.) 



FIG. 709. Clinos- 



