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. CcphaUc glands in ixjstfrior third 

 of body. A pair of flame cells (in pockets) in posterior third of excretory trunks. Eyc-sp«jls 

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

 to acetabulum. Tail about twice body length, furcae of same length as undivided ix^rtion. In 

 long attenuated sporocysts in Hver 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 refiexed, 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 24S 



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



248 (249) Develop in rediae or unbranched sporocysts. 



Cercariacum. 



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. 



Cercariacum hclicis (Leidy) 1847. 



Total length 0.85 mm.; breadth 0.6 mm., active and vcr>' 

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

 marked by radial Hues; 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. Oenital pore ix)stacetal)ular. 



In pericardial cavity of Helix allcrtiala and //. albolabris. 

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

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

 the name Disloma hclicis was originally given. 



Called later D. vagans al.st) by Leidy. Die.sing makes it 

 Cercariacum vagans. Possibly a cercaria which has thrown 

 off its tail but has not encysted. 



Fig. 729. Cercariaeum Ittlicis. Second stage; hiRhly magnific<I. 

 (.\fter Lciiiy.) 



249 (248) Develop in branching sporocysts Lcucochloriiiium 



The remarkable species is known in Europe in the adult ft)rm as a fxirasite of sin^ng birds 

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

 living period. 



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



tal atrium modified into sucking organ. 



I loJostome cercariae . . 251 



Genital opening posterior, ventral to excretory pore. 



