PARASITIC FLATWORMS 



423 



243 (242) Cephalic glands open into ducts posterior to acetabulum. . . 244 



244 (245) Cephalic region crowned with spines; two eye-spots present. 



Cercaria gracillima Faust. 



Body length 0.13 to 0.16 mm., width 0.02 to 0.03 mm. Cephalic glands in posterior third 

 of body. A pair of flame cells (in pockets) in posterior third of excretory trunks. Eye-spots 

 lying directly lateral to cephalic ganglia, unpigmented. Genital rudiment extends anterior 

 to acetabulum. Tail about twice body length, furcae of same length as undivided portion. In 

 long attenuated sporocysts in liver of Physa gyrina Say, near Buckhouse Bridge, Bitter Root 

 River, Montana. 



245 (244) Cephalic region crowned with two small tubercules; eye-spots 



lacking Cercaria tuberistoma Faust. 



Body length 0.2 mm., width 0.05 to 0.06 mm. Tail about 0.32 mm., furcae equal in length to 

 undivided portion. Cephalic glands small; excretory system simple, most anterior tubules of 

 tail reflexed, bladder muscular. In sporocysts, either dumbbell shaped or attached at one 

 end. No birth pore. Cercariae escape by splitting wall of sporocysts. In liver tissue of 

 Physa gyrina Say, Bitter Root River, Corvallis, Montana. 



246 (241) Modification consists of lateral spines in rows. 



Setiferous cercariae. 

 None yet recorded from North America. A small group, mostly marine. 



247 (191) Tail apparently entirely wanting. . -. 248 



The tail may be small and easily lost or actually not developed. 



248 (249) Develop in rediae or unbranched sporocysts. . . . Cercariaeum. 



The young distomes possess no cyst or protective mem- 

 brane. Found not infrequently in our fresh-water mussels. 

 Species not described. Adults unknown. One of the Euro- 

 pean species is thought to be the larva of Asymphylodora. 



Cercariaeum helicis (Leidy) 1847. 



Total length 0.85 mm.; breadth 0.6 mm., active and very 

 extensive. Body white, oval, with oval tail. Oral sucker 

 marked by radial lines; acetabulum central, equal in size to 

 oral sucker, 0.15 mm . in diameter. Pharynx oval. Intestine 

 large, sinuous, extending to end of body. Excretory bladder 

 small; lateral vessels double. Genital pore postacetabular. 



In pericardial cavity of Helix alternata and H. albolabris. 

 The "first" and "third" stages of Leidy's later account are 

 clearly not the same species as the "second stage" to which 

 the name Distoma helicis was originally given. 



Called later D. vagans also by Leidy. Diesing makes it 

 Cercariaeum vagans. Possibly a cercaria which has thrown 

 off its tail but has not encysted. 



FIG. 729. Cercariaeum helicis. Second stage; highly magnified. 

 (After Leidy.) 



249 (248) Develop in branching sporocysts Leucochloridium 



The remarkable species is known in Europe in the adult form as a parasite of singing birds 

 and in the sporocyst stage in certain snails, Succinea. See 157 in this key. It has no free- 

 living period. 



250 (184) Oral sucker rudimentary, much smaller than acetabulum. Geni- 



tal atrium modified into sucking organ. 



Holostome cercariae . .251 

 Genital opening posterior, ventral to excretory pore. 



