70 ILLINOIS BIOLOGICAL MONOGRAPHS [70 



The cercaria was found in the liver tissues of Physa gyrina Say from the 

 Bitter Root River in the vicinity of Hamilton, Montana, in October 1916. 

 Forty per cent of the physas examined were infected with the parasite. The 

 cercaria develops within a very simple sporocyst, which has a length of 0.34 

 mm. and a width of 0.17 mm. (Fig. 67). The wall of the sporocyst is delicate, 

 consisting of a single layer of very thin epidermal cells, with no basement mem- 

 brane and no muscular complement. The genital epithelium is localized at one 

 end, and from this only a few cercariae are developed at any one time. 



The excretory system of Cercaria glandulosa presents some interesting 

 features. The bladder is flattened, truncate, and subterminal instead of 

 terminal. A narrow canal communicates with the excretory pore which is 

 median posterior. The four angles of the bladder are muscular. When the 

 bladder is emptied these corners He close together, so that the cavity of the 

 bladder is small. Then by the expansion of the bladder this organ is filled 

 from the trunks (Figs. 64, 65). Two vesicular cornua empty into the bladder. 

 Each cornu is directed laterad and slightly anteriad; it soon constricts to 

 form the lateral tube. The common tube divides soon to form the posterior 

 tubule and the anterior tube. The anterior vessel then divides in the region 

 of the acetabulum to form a trifurcate system. Just behind the region of 

 this division there is a small vesicular swelling where granules of the system 

 accumulate. The excretory system in the tail consists of the common medi- 

 an vessel and several tributaries. 



The digestive system is characterized by an abundance of glands, so that 

 the entire tract is surrounded with gland cells. A small pharynx surrounds 

 the esophagus near the anterior end of the tube. The esophagus extends to 

 the preacetabular region, at which place it forks to form short furcae which 

 barely clasp the anterior end of the acetabulum. Along this entire course 

 there are many gland cells in clusters, especially abundant in the pharynx 

 region. Their relation to the pharynx and nerve ganglia is shown in figure 72, 

 The individual gland cell is ovate, with a short neck. The cytoplasm is 

 chromophilous. There is no recognizable duct connection thru the myoblasts 

 of the pharynx to the lumen. The nuclei of these cells are large and studded 

 with granules. 



In addition to the grape-like clusters of gland cells surrounding the entire 

 digestive tract there are right and left paired gland groups of the salivary- 

 mucin type. They consist of nine large cells to each group, usually situated 

 in the acetabular region, but capable of extension, so that they may lie as 

 far caudad as the bladder (Fig. 62). Figures 73 and 74 show sections passing 

 thru the anterior tip of the excretory vesicle. In each of these a right and a 

 left gland are visible. In these glands not only is the nucleus granular, but 

 the cytoplasm is densely granular, the granules being assembled in little 

 clumps. Frequently (Fig. 73) there are vacuoles witliin the cytoplasm. 



The genital organs are represented by cell masses which show clearly the 

 location of the mature organs, but as yet show little differentiation (Fig. 



