76 ILLINOIS BIOLOGICAL MONOGRAPHS [76 



end are six gland cells, paired right and left. They appear as small tubercules 

 suspended from the anterior wall of the vesicle. Anterior to the median vessel 

 are two cornua, elongate, yet swollen, reaching antero-laterad around the 

 acetabulum. Near the acetabulum there are received the common posterior 

 and the two anterior tubules. The pattern of the capillaries is racemose. 

 The tail trunk system consists of a common median vessel with many lateral 

 tubules. The entire system is filled with minute excretory granules. 



The digestive system consists of a very long esophagus, near the anterior 

 end of which is the small sphincter, and from the posterior end of which the 

 furcae arise. They extend partly around the acetabulum. The salivary-mucin 

 glands consist of right and left paired groups of cells, eight to the group, with 

 long ducts leading in a common bundle to the oral pocket. 



In the region of the cerebral ganglion a pair of oval bodies, the non-pig- 

 mented eye-spots are located. They are degenerate, similar to those eyes 

 described for Cercaria gracillima (p. 52). 



The genital cell masses are found in the region of the acetabulum (Figs. 

 104, 107). To the left is Laurer's canal, and running dextro-laterad is the 

 closely coiled uterus. The genital pore is on the right of the mid-ventral line, 

 anterior to the acetabulum. Running into the ootype from the postero- 

 lateral angles are the vitelline ducts, connecting the vitellaria with the ootype. 

 The testes are not well defined. The relationship of the cercaria is not evi- 

 dent from the genital cell masses. 



Cystogenous cell glands are present, altho not as conspicuous as in C. 

 glandulosa or C. micropharynx. Encystment takes place after a considerable 

 period of free swimming life. Decaudation always precedes encystment. 

 The cyst wall is thin; the animal is easily viewed thru the cyst. 



Echinostome Cercariae 

 Cercaria trisolenata Faust 1917 



Cercaria trisolenata represents a unique type of echinostome larva (Fig. 

 109). It is more attenuate than the average species of this family, and has 

 an unusually short tail. The body has an average length of 0.45 mm. and a 

 width at the preacetabular region of 1 mm. The tail is about 0.2 mm. long, 

 lanceolate, and measures 0.06 mm. at the base. An anterior region of the 

 trunk, measuring 0.06 mm. along the median line, constitutes the head region, 

 behind which is a neck-like constriction. There is a collar of 36 spines along 

 the margin of the head, arranged in a single irregular series (Figs. 110, 111). 

 These spines are bluntly rounded at the base and taper to a rounded point 

 at the distal end. They are from 12/1 to 14/* in length. The body as a whole 

 is usually covered with minute spines. The acetabulum is beset with an 

 irregular arrangement of crooked spines (Fig. 112). The oral sucker is small 

 but powerful, 33yu in diameter. The acetabulum, situated behind the mid- 

 plane of the body, measures 42/c. 



These cercariae, together with Cercaria gracillima, are the most cosmopoli- 

 tan species of the Bitter Root River. They occur in Physa gyrina from the 



