74 ILLINOIS BIOLOGICAL MONOGRAPHS [74 



Soon after the cercaria is set free into the water, it drops its tail. Ahnost 

 before the observer is aware it secretes a thin membrane from the abundance 

 of cystogenous material contained in the large cyst cells which pack the paren- 

 chyma of the worm. The oval c>'st with the worm coiled up inside is shown 

 in figure 84. This type of cyst offers only a temporary lodgement for the 

 cercaria, and it is evident that the worm must reach the definitive host soon 

 if the infection is' to be successful. 



Cercaria micropJiarynx Faust 1917 



Cercaria micropJiarynx is a minute larva of the xiphidiocercariae, oval in 

 contour, with small clavate tail (Fig. 93). The body is covered with minute 

 spines arranged in diamond pattern, progressively less prominent toward 

 the caudal end. The spines are probably constant characters of the adult 

 as weU as of the larva, since the entire trunk is well suppHed with these spines 

 while the tail is naked. The body measures 0.18 mm. in length and 0.09 mm. 

 in width. The tail is 0.14 mm. long and 0.03 mm. at the base. It is inserted 

 into a caudal pocket provided with a group of spines on the lateral lappets 

 ventral to the tail. The oral sucker is large for the bod}'- size, 35^1 in diameter, 

 while the acetabulum is shghtly smaller, 30/i in diameter. Inserted in the 

 hood of the oral sucker is the stylet organ (Figs. 91, 92), 34fi long and 5n to 

 6fi in breadth along the shaft. The organ is reinforced all around and has a 

 velum stretched across the ventral surface of the quiU. 



The cercaria was secured from the infected Uver tissues of a large number 

 of Lymnnea proxima, taken from Rattlesnake Creek, Missoula, in Novem- 

 ber 1916 and in May 1917. The cercariae develop in oval irregular sporo- 

 cysts, measuring 0.24 mm. along the long axis and 0.18 mm. along the short 

 axis (Fig. 94). The body wall of the sporocyst consists of a single layer of 

 epidermal cells, between which are found numerous excretory granules, lying 

 in irregular grooved channels. There is no localization of the germinal epi- 

 theUum, so that germ balls arise from aU portions of the body wall and, when 

 mature, break out into the body lumen. Not only do the cercariae develop 

 to maturity in the sporocyst, but in some cases they drop their tails and 

 encyst in the sporocyst (Fig. 95). Thus the larval host, the snail, is the food 

 of the intercalated or of the definitive host, since no free-living stage is com- 

 monly found. In case the cercaria is pressed out of the sporocyst before 

 encystment, it swims about for a very brief period, then drops the tail and 

 enc>^sts. 



The excretory system consists of a subspherical vesicle and bellows-shaped 

 comua, which open into the vesicle thru a common cylinder. The three 

 usual tubes of the excretory system are present, the single posterior and the 

 two anterior ones. The tail tube is single median, with a few inconspicuous 

 lateral tributaries. The comua are filled with excretory fluids; they are 

 Imed with cells (Fig. 98). 



The digestive system consists of the very minute pharynx in the mid- 

 region of the esophagus, and two vesicular f urcae considerably anterior to the 



