PARASITIC FLATWORMS 421 



231 (232) Tail simple, unmodified Cercaria triwhis Cort 1914. 



Both rediae and cercariae in Planorbis trivolms, Urbana, Illinois. 

 Moves actively in free water and on solid bodies; found encysted in 

 same host with rediae and cercariae. Nuclei of sex organs in two 

 masses, connected by slender thread. Collar carries thirty-seven equal 

 spines in two alternating rows; two or three spines near mid- ventral line 

 point inward. 



FIG. 726. 



Cercaria trivolms, mature, ventral view. Cystogenous glands not 

 shown. X 65. (After Cort.) 



232(231) Tail with lateral fin-folds Cercaria reflexa Cort. 



Though without oral spines in the stage originally discovered and described this species 

 probably belongs here among Echinostomid cercariae. 

 For description see 197 (198) in this key. 



233 (194) Tail simple but heavy, when contracted exceeding in breadth the 



body . Rhopalocercous cercariae. 



Listed from North America. 



Cercaria (Rhopalocerca) tardigrada Leidy 1858. 



Reported by Leidy from Anodonta species. The true R. tardigrada is Dist. duplicatum v. 

 Baer renamed and is the larva of Phyllodistomum folium according to Liihe. Perhaps Leidy's 

 form is the larva of some American species in that genus. 



No North American cercariae have yet been well described which fall into this subdivision 

 though both of the species listed by Liihe for Central Europe belong to genera, Attocreadium 

 and Phyllodistomum, which are reported here. These are not closely related genera and the 

 group of cercariae does not appear to be a natural one as at present constituted. 



Note that Odhner believes that the larva of Phyllodistomum folium occurs in bunches, as 

 stated in 189 (186) of this key. 



234 (192) Tail well developed and highly modified 235 



235 (240) Base of tail envelops body of young distome. 



Cystocercous cercariae . . 236 



The anterior end of the tail is expanded in the form of a bladder into which is folded the 

 body of the young distome that lies thus in a sac or chamber. 



236 (237) Chamber globular, small. Tail simple, slender. European type. 



Cercaria macrocerca Filippi 1854. 



These forms of which several have been described in Europe are the young forms of Gorgo- 

 derinae (no in this key). The adults have been reported from this country, but this larval 

 form is yet to be identified here. 



2 37 (236) Chamber large; round. Tail flat, forked, anchor-shaped with 



broad terminal flukes; powerful swimming organ. . . 238 



These move by rapid alternate lateral jerks of the anchor flukes. As the distome is thus 

 pulled along by the tail, the usual orientation of a cercaria is reversed. The adults are unknown. 



