479] LARVAL TREMATODES—CORT 33 



cavity depending on the amount of food material present and the 

 pressure from the developing cercariae. In the inner lining of the 

 intestine were found flattened, scattered nuclei, but no cell boundaries 

 could be distinguished. The body cavity occupies most of the entire 

 region from just back of the oral sucker up to the posterior tip, but 

 does not extend into the posterior locomotor appendages and the tail-like 

 posterior extremity. These regions are filled with parenchymatous 

 tissue in which definite cell boundaries could be determined (Fig. 35). 

 The wall of the body cavity of even the youngest redia is very thin, and 

 in the inner lining of flattened pavement cells with flattened nuclei, the 

 cell boundaries could be distinguished only with difficulty. A small 

 germ gland consisting of but few differentiated cells is present at the 

 posterior extremity of the body cavity. 



All ages of rediae were present in the snail from those in which 

 the oldest contained cercaria was scarcely differentiated at all, to those 

 in the body cavities of which there were from four to eight almost fully 

 matured cercariae. The youngest redia studied (Fig. 36) which con- 

 tained no cercariae having the cystogenous glands at all developed, was 

 0.53 mm. in length and 0.12 mm. in greatest width. The posterior loco- 

 motor appendages were 0.097 mm. from the posterior end, and the 

 intestine extended to within 0.076 mm. of the posterior extremity. The 

 pharynx was slightly elongated, being 0.049 mm. in length and 0.043 

 mm. in width. The body cavity contained a number of developing 

 embryos only one of which was far enough along to be recognized as a 

 cercaria. In this embryo (Fig. 36, cv) the tail was a mere stub hardly 

 marked off from the body. The length of the body was 0.33 mm. and its 

 width 0.054 mm., while the tail had a length of 0.038 mm. and a width 

 of 0.032 mm. The sucker, digestive system, and reproductive anlage 

 could be made out. 



From one of the largest rediae studied the following measurements 

 were taken (Fig. 31). The length was 1.16 mm., and the width 0.19 

 mm. The posterior locomotor appendages were 0.27 mm. from the pos- 

 terior extremity, and the intestine, which filled about half the body 

 cavity, extended to within 0.086 mm. of the posterior tip. There were 

 four fully matured cercariae, one of which while still in the redia had 

 extruded its cystogenous material, and a number of developing embryos 

 of all ages. The pharynx of this redia was 0.054 mm. in length, and 

 0.049 mm. in width, showing almost no development in size from the 

 very youngest redia. 



The length and width of the body of Cercaria megalura (Figs. 29 

 and 30) vary greatly with the state of contraction. The tail varies 

 from one-half the length of the body when the animal is moving on a 



