412 



FRESH-WATER BIOLOGY 



172 (173) Mouth opening some distance from anterior end, near center of 

 ventral surface. Intestine rhabdocoel. 



Gasterostomous or rhabdocoelous cercariae. 



Alimentary canal short, simple, rod-shaped. Swimming organ in the form of two long 

 narrow appendages directed obliquely right and left from posterior end of body in the only 

 known type. Larval stages of Bucephalidae, such as the well-known Bucephalus polymorphus 

 von Baer of Europe. 



Not yet reported on this continent though the adult (see 29 in this key) is known here. 



i?3 (172) Mouth opening at or very near anterior end of body. Intestine 

 triclad Prostomatous cercariae . . 174 



174 (183) Only one sucker, and that around mouth opening. 



Monostome cercariae . . 175 



All yet studied have a pair of lateral pigment spots on the dorsal surface, the simple eyes. 

 Some have also a medium pigment area between the lateral eyes or slightly anterior to them. 



175 (178) Cercariae without median eye in cephalic region 176 



Under the designation "median eye" is included always an optic cup with pigment lining 



and an optic cell; part of these are found in certain developmental stages but in species in- 

 cluded under this heading disappear so that the structure never becomes complete. 



176 (177) Six pairs of large gland cells in tail. 



Cercaria urbanensis Cort 1914. 



Length 0.27 to 0.54 mm., width o.n to 0.2 mm., tail 0.2 to 1.2 mm. long and 

 0.05 mm. at base. Develops in rediae. An active swimmer. Encysts on solid 

 objects. Cysts shaped like thick discs. Moves over surface by aid of two pro- 

 jections one at each postero-lateral angle of body and with cuticular knob in 

 tip. Heavily pigmented, especially near anterior end. A pair of lateral eyes; 

 intermediate pigment nucleus present in later stages, but no optic cup or cell at 

 any time. Cystogenous glands abundant. From Physa gyrina at Urbana, 

 Illinois. 



FIG. 713. Cercaria urbanensis, mature, ventral view. Cystogenous glands not shown. 

 X 70. a, posterior locomotor projection. X 216. (After Cort.) 



177 (176) Six groups of paired gland cells in tail, each pair dove-tailing into 



the one next anterior Cercaria konadensis Faust. 



Cercaria 0.4 to 0.46 mm. in length, o.i to 0.16 mm. in width. Tail 0.4 to 0.45 mm. long by 

 0.03 to 0.04 mm. diameter at base. This species possesses no median pigment area in cephalic 

 region. Glands of posterior locomotor organ large and prominent. Cercariae and rediae 

 aspinose. Germ balls arise from central germinal rachis in subdistal region of redia. 



From liver of Lymnaea proximo, Lea, Bitter Root River, Corvallis, Montana. 



178 (175) Cercariae with median eye or median pigment area in cephalic 



region. Larger species than preceding. . ...... 179 



179 (180) Distinct mobile, evertible spinose pharynx. 



Cercaria pelliicida Faust. 



Length 0.4 to 0.7 mm., width 0.18 to 0.2 mm. Tail 0.5 mm. in length, 0.07 mm. in diam- 

 eter at base. No large gland cells in tail. Rediae provided with multispinose evertible 

 piercing organ in prepharynx region. Germ balls arise from central germ cells in distal 

 region of redia. From liver interstices of Physa gyrina Say, Bitter Root River, Fort Missoula 

 and Corvallis, Montana. 



180 (179) No evertible spinose pharynx mentioned (imperfectly known 



species) 181 



