40 ILLINOIS BIOLOGICAL MONOGRAPHS [486 



pushed ventrad by the cercariae, extended for more than one-third of 

 the length of the body. 



The pharynx is about the same size in the youngest and the oldest 

 rediae, and the actual size of the intestine differs but little, altho its 

 ratio to the size of the body is much less in the older form. The birth- 

 pore is located on the dorsal side of the body a little back of the pharynx. 

 In the oldest rediae the anterior collar is not present and the posterior 

 locomotor appendages are much reduced. 



Cysts of Cercaria rubra (Fig. 41) were present in the tissue above 

 the gills of six of the thirty-six specimens of Campeloma stibsolidum 

 from Hartford, Connecticut. The cysts were large, round, thick-walled, 

 and of very uniform size, measuring 0.195 mm. to 0.205 mm. in diame- 

 ter. The cyst wall was transparent and had a thickness of 0.016 mm. 

 The worm almost completely filled the cyst with practically the whole 

 dorsal surface against the wall, and the posterior end overlapping the 

 anterior. 



Several cysts were opened and the worms freed. A study was 

 made of these while living but none were preserved. 



The living cercariae (Fig. 40) were on the average 0.50 mm. in 

 length and 0.15 mm. in width at the region of the heart-shaped anterior 

 end. They tapered slightly posteriorly, had a width at the acetabulum 

 of 0.13 mm. and the end was bluntly rounded. 



The oral sucker of Cercaria rubra was a most exactly round having 

 a transverse diameter of 0.043 mm. and the acetabulum which is two- 

 thirds of the distance from the anterior to the posterior end was larger, 

 measuring 0.065 mm. 



The collar which is typical of the Echinostomes is very well defined 

 in this species and has arranged around its edge in two alternating rows, 

 forty-three spines, which vary only from 0.018 mm. to 0.022 mm. in 

 length. In the middle of the ventral surface as is usual there is a 

 depression and a break in the rows of spines. The four median spines 

 on each side of this space are not in regular line with the others and 

 point inward. Besides these eight there are seventeen spines in the 

 upper row and eighteen in the lower row (Figs. 40 and 42). The 

 surface of the body as far back as the acetabulum was covered thickly 

 with rows of spines pointing backward. They were 0.005 to 0.007 mm. 

 in length. The rows were 0.008 mm. apart and the spines were set 

 thickly in the rows. 



The digestive system (Fig. 40) offered nothing peculiar. The pre- 

 pharynx had a length about equal to the diameter of the oral sucker, 

 and the pharynx had a diameter of 0.025 mm. The short esophagus and 



