493] LARVAL TREMATODES—CORT 47 



and set back slightly from the anterior end so that the anterior part 

 of the oral cavity is in front of it. The intestine is narrow and elongate 

 reaching nearly to the end of the body and almost filling the small 

 body cavity. Inside of the cuticula and muscle layers the wall is made 

 up of embryonic nuclei with poorly defined cell boundaries. There 

 are present no developing embryos and the germ gland is not clearly 

 differentiated. 



Very few rediae were found showing intermediate stages between 

 the small type just described and the largest forms. The one shown 

 in Figure 53 with only five germ balls of any size in its body cavity is 

 0.43 mm. in length and 0.09 mm. in diameter. The pharynx has the 

 same width as length, 0.043 mm., and the pouch shaped intestine, which 

 is 0.11 mm. long and 0.038 mm. at its greatest width, extends to a point 

 only about one-third of the distance from the anterior to the posterior 

 end. The body cavity is large and contains oval germ balls, the largest 

 of which is 0.076 mm. long and 0.06 mm. wide. The wall of the cavity 

 is about two cells thick and the germ gland is clearly defined con- 

 taining both single germ cells and balls of several cells. The pharynx 

 in this form is also set back from the anterior end. 



The largest rediae (Fig. 52) are all rounded at their posterior 

 extremities and their body cavities are very large with thin walls. The 

 germ gland is much reduced. At one side near the anterior end is located 

 the birth pore with protruding lips. The rediae vary in size up to the 

 largest measured which is 0.73 mm. in length. The largest ones have 

 from 25 to 30 germ balls of about the same size in their body cavities. 

 The intestine is smaller in proportion to the size of the body but of 

 about the same absolute size as in the smaller forms, and the oral sucker 

 is very clearly set back from the anterior end. The outer layers at the 

 anterior part are wrinkled so that the appearance is given of a series 

 of horizontal folds from the anterior tip back to the middle of the body. 



Besides the large rediae with the body cavity full of germ balls 

 there are present numbers of large rediae with few or no germ balls and 

 much constricted and twisted. 



That these rediae are the nurse generations of Cercaria trigonura 

 is very probable from their being found in every instance with that 

 form. All the most immature rediae are about the same size and appear 

 to belong to the same brood, yet there is no evidence as to their origin. 

 Neither is there any evidence where the cercariae free in the tissues 

 come from, for in none of the rediae can cercariae be distinguished. 

 The old rediae shells may have been the nurses of these cercariae and 

 rediae but even when a few embryos are found in these, they are merely 

 large little differentiated germ balls. It is possible that in these snails 



