459] LARVAL TREMATODES—CORT 13 



The rediae of Cercaria urbanensis (Figs. 10, 11, and 14) were in 

 various stages of development, varying in length from 0.52 mm. to 

 1.08 mm. and in width from 0.12 mm. to 0.22 mm. In shape they 

 are elongate sac-like forms smallest at the anterior and widest a short 

 distance in front of the posterior extremity. The mouth is at the an- 

 terior tip and the pharynx varies with the size of the animal. In the 

 smallest redia studied which had a length of 0.52 mm. the pharynx 

 was 0.038 mm. in length by 0.048 mm. in width while in one 1.08 mm. in 

 length it was 0.07 mm. by 0.08 mm. The passage thru the pharynx 

 immediately opens into the intestine which is at first narrow but soon 

 widens greatly. In all stages of development the intestine is propor- 

 tionally very large, having a diameter of from one-third to two-thirds 

 the width of the body and reaching to within 0.08 mm. to 0.16 mm. 

 of the posterior end. It is rather clear and transparent and contains 

 only a small amount of food material floating in a considerable quantity 

 of fluid. The outer cuticula of the redia is very thin and can hardly 

 be distinguished as a separate layer. Inside of this is a very strong 

 layer of circular muscles which encircle the body as separate strands, 

 each about 0.0018 mm. in width and about the same distance apart. 

 The strands of the longitudinal layer are very thin and only visible 

 under favorable conditions. Inside of the muscle layers are several 

 rows of parenchymatous cells with rather large nuclei. These cells do 

 not form a circumscribed lining of the body cavity but are irregular 

 and in young rediae where the lining is several cells thick, strands 

 extend from them thruout the body cavity forming an irregular net- 

 work (Fig. 13). Into this loosely filled space between the body wall 

 and the intestine the cercariae push. In the oldest rediae the wall 

 has been reduced to one layer and the movements of the cercariae have 

 broken down the parenchymatous strands and converted the region 

 between the wall and the intestine into a well defined cavity, which 

 is more or less completely filled with developing cercariae. In the 

 posterior end of the redia is the germ gland, in front of this are germ 

 balls and further forward in the older rediae are differentiated cercariae. 

 In the youngest redia (Fig. 10) studied the developing embryos were 

 all back of the middle of the body and the furtherest developed was a 

 mass of embyronic cells about 0.09 mm. in length and 0.06 in width 

 and having no tail. In none of the rediae was the body cavity crowded 

 with cercariae, there being but two or three well developed forms, and 

 in one mature specimen there were no differentiated cercariae present 

 leaving the body cavity empty for about two-thirds of its length. These 

 conditions and the fact that the oldest cercariae in the rediae are not 

 fully mature and that matured and almost matured cercariae are found 



