80 ILLINOIS BIOLOGICAL MONOGRAPHS [80 



into inner and outer series. The condition is similar in part to that in Cer- 

 carta crenata (Fig. 55), in which species there is also a double series of glands 

 on each side of the esophagus. But in this case the glands of the inner and 

 outer series are not different in structure and function as in C. crenata. Here, 

 too, the ducts are not composed of a bundle of separate ducts, as is found in 

 C. marcianae (La Rue, 1917:5), but consist of a single common duct for the 

 entire group of gland cells. 



The genital system is much further developed in this species than in C 

 trisolenata (Fig. 139). Behind the acetabulum is the ovary, from which a 

 uterus leads around the acetabulum, ending in a swoUen vagina in the pre- 

 acetabular region. Only the transverse ducts of the vitelline system are 

 differentiated. Behind these are the two testes, unequal in size, one above 

 the other. 



The encystment of Cercaria hiflexa is similar to that of C. trisolenata in 

 that it depends on the secretion of mucoids from a large number of .cystogenous 

 glands in the parenchyma. It differs, however, in the two species both as to time 

 and place of encystment. Cercaria trisolenata encysts only after it has escaped 

 from the liver tissues of the host. C. biflexa encysts within the host, immedi- 

 ately upon breaking thru the birth-pore of the redia. Thus a section of 

 Planorbis trivolvis liver tissue shows the interstices of the liver ceca filled with 

 encysted cercariae, which continue to grow and differentiate within the pri- 

 mary host (Fig. 159), 



Furcocercariae 

 Cercaria gracillima Faust 1917 



Cercaria gracillima is a furcocercous cercaria more slender than those 

 previously described. This species, together with C. tuberistoma (p. 82), 

 constitute the second instance of furcocercous cercariae to be described for 

 North America, the first being C. douthitti Cort (1915:50-52; Figs. 55-64), 

 In addition to the forked-tail character of these three species, they lack a 

 pharynx, and have paired salivary-mucin glands leading into the oral pocket, 

 composed of four or more cells to each group. The "eyelet" excretory ana- 

 stomosis, connecting the excretory system of the body and the tail is also a 

 common character. 



Cercaria gracillima has an oblong cylindrical body (Fig. 142). The 

 branched portion of the tail is elongate lanceolate. The body length varies 

 from 0.13 mm. to 0.16 mm. and the diameter of the worm varies from 0.02 

 mm. to 0.03 mm. The unbranched portion of the tail is about 0.16 mm. long 

 and the caudal rami are of equal length. The former is 0.02 mm. to 0.03 mm. 

 in diameter and the ramus has a width of 0.01 mm. The trunk is characterized 

 further by the absence of a true oral disc, while in its place there is an invertible 

 sucker. A ventral sucker, 12/i in diameter, varies in location, according to 

 the movements of the animal, from the middle of the ventral side to a position 

 considerably farther forward. The cephalic region is ovately rounded when 

 the sucker is fully distended and is crowned by a cap of small spines. A 



