PARASITIC FLATWOKMS 



415 



193 (233) Tail slender, never as broad as body of cercaria. . 



Cercariae leptocercac . . 194 



The long, slender, unbranched tail, which even in maximum contraction does not reach the 

 width of the body, and in extension is twice the body length or more, is found in the majority of 

 distome cercariae. The anterior end of the body furnishes data for the subdivision oi these 

 forms. 



194 (199) Anterior end rounded, entirely devoid of spines. 



Gymnocephalous cercariae . . 195 

 So far as known these forms develop in rediae. Many exist on this continent which have 

 not been reported, for many adults are listed in the earlier sections of this key which must 

 possess such larvae as one may infer from European studies on related species. These ceitariat 

 are conveniently subdivided on the structure of the tail which in all is a prominent oigan but 

 which in some does not function as a swimming organ. 



195 (196) Tail simple, not provided with fin-folds or terminal sucking organ. 

 Three different forms may be noted without attempting to analyze them in the key 



Cercaria {Gymnoccphala) ascoidca (Leidy) 1877. 



Length 0.25 to 0.4 mm. Body, white clavate; tail long, narrow, cylindrical pointed 

 Cephahc end triangular and slightly constricted from rest of body. Acetabulum at or behind 

 center of body often protruded into a cone or expanded into a cup. Xo eyes. In movement 

 excessively elongated. Rediae white; head distinct from cylindrical body, with birth pore 

 and caudal prolongation. 



Abundant in Planorbis parvus and found free in water containing that species. Leidy is in 

 error in identifying this form as Cercaria minuta Nitzsch of Europe. 



Cercaria agilis Leidy 1858. 



Body pyriform, oral sucker large, acetabulum slightly larger, near middle of body. Tail as 

 long as body, clavate, transversely pHcate. White. Very active. 

 Found in Delaware River; ordinarily with snails. Common. 



Cercaria fasciolae hepaiicae. 



Larva of the well-known sheep liver fluke, not yet reported but undoubtedly frecjuent in 

 certain regions and years as the adult is known to be abundant at certain ix)ints in North 

 America. 



196 (195) Tail modified, having fin-folds or terminal organ. 



197 



197 (198) Tail provided with dorsal and ventral fin-folds. 



Cercaria rcflexa Cort 19 14. 



Develops in rediae. Encysts in same snail as redia inhabits or other 

 snail of the same species. Tail as long as body or longer, provided 

 with dorsal and ventral fins. Cystogenous glancls abundant. Es<^)ph- 

 agus long, fine; alst) crura; bifurcation at anterior level of acetabu- 

 lum. Genital organs marked out by four masses of nuclei. Cort 

 believes this form is undoubtedly related to the Echinostomcs, but 

 the spines are not yet developed. .Ml other characters accord with 

 this view. From Lymnaea rcjlcxa, Chicago, Illinois. 



Compare 232 (231) in this key 



Fig. 717. Cercaria rcjlexa, ventral view. Cvstogcnous glands not showi 

 X 60. (.\ftcr Cort.) 



