46 ILLINOIS BIOLOGICAL MONOGRAPHS [492 



Set in the dorsal part of the oral sucker and protruding for about 

 one-fifth of its length is a cephalic spine or stylet (Figs. 47, 49). The 

 spine has a length of 0.018 mm. and a width of 0.004 mm., is sharply 

 pointed and slightly thickened about two-thirds of the distance from 

 the base to the tip. Ventrally no thickening is present. 



The cephalic or stylet glands (Fig. 48, sg) fill the space 

 dorsad and anteriad to the acetabulum and extend forward to just back 

 of the nervous system. The individual glands are small, averaging 

 0.024 mm. in length and 0.012 mm. in width, finely granular and con- 

 tain small nuclei. Their ducts pass forward dorsally over the oral 

 sucker and open around the cephalic spine in the anterior pit. The 

 numbers were so large that no accurate count could be made. They 

 form a single mass unbroken in the median line. 



The mouth is subterminal and back of the oral sucker is a small 

 pharynx, 0.012 mm. in length by 0.01 mm. in width. The prepharynx 

 is short and the esophagus and intestinal ceca are entirely undeveloped. 



The excretory pore of Cercaria trigonura opens dorsally at the 

 posterior extremity of the body just at the base of the tail. Leading 

 up to it is a narrow tube near the dorsal surface which expands into 

 the large bicornuate excretory vesicle, the horns of which reach on each 

 side of the acetabulum. Vessels could be traced along the sides of the 

 body from the tips of the horns of the vesicle up to the pharynx. 

 Figures 48 and 50 give the relations of the pore vesicle and vessels. 

 The vesicle is lined with a thick layer of granular, cuboidal, epithelial 

 cells with large nuclei (Fig. 54, ex). An excretory vesicle with thick- 

 ened walls of the character just described has been found in the other 

 so-called stumpy-tailed forms. 



The anlage of the reproductive organs shows as a mass of nuclei 

 dorsal to the posterior part of the acetabulum (Fig. 48, ra). 



Along with the cercariae in the tissues of Campeloma siibsolidum 

 were large numbers of rediae of different sizes. They were found in 

 every snail infected with Cercaria trigonura and were not found in any 

 instance where this species was not present. The only rediae that 

 showed any activity were very small immature forms which were present 

 in considerable numbers and in the same stage of development. They 

 were active, extending and contracting and twisting and turning in 

 all directions. Part of them were sharply pointed and the others bluntly 

 rounded posteriorly. The first type (Fig. 51) was the most common 

 and the structure of these will be described in some detail. The avera- 

 age of the mounted specimens of the small rediae is 0.019 mm. in length 

 and 0.04 mm. in width. The pharynx is small, 0.03 mm. in width 



