485] LARVAL TREMATODES—CORT 39 



round ones. At the region of the pharynx the excretory tubules are 

 much smaller and curve around forming a characteristic loop, from 

 which a small vessel on each side could be traced back to the level of the 

 acetabulum. 



The nuclei which form the anlage of the reproductive organs of 

 Cercaria trivolvis are not yet fully divided into separate masses. In 

 the midline of the body just in front of the excretory bladder and 

 behind the acetabulum is an elongate mass of these nuclei from which 

 a line can be traced forward to a smaller mass in front of the acetabu- 

 lum. The posterior mass probably develops into the ovary and testes, 

 and the anterior mass represents the ends of the reproductive ducts. 



Eediae of Cercaria trivolvis (Figs. 37 and 38) were present in the 

 infected snails in various stages of development. The youngest were 

 unpigmented but in the older ones orange colored pigment had devel- 

 oped in the outer wall, which rendered the largest rediae almost opaque. 



The smallest redia found had a length of 0.30 mm. and a width of 

 0.065 mm. The posterior locomotor appendages were 0.22 mm. from 

 the anterior end and the region back of them was attenuated and 

 pointed. The oral sucker had a length of 0.041 mm. and width of 0.043 

 mm. and the intestine, which was very narrow and elongate, reached to 

 the posterior locomotor appendages. No germ balls were present in the 

 body cavity which was but little larger than the intestine, and the region 

 of the germ gland was not visibly differentiated. 



In a redia (Fig. 38) 0.41 mm. in length and 0.081 mm. in width 

 the posterior locomotor appendages were 0.34 mm. from the anterior 

 end. A ridge extended around the body like a welt 0.086 mm. from 

 the anterior end. The body cavity at this stage had become well devel- 

 oped and contained germ balls, none of which however were in front of 

 the posterior extremity of the intestine. The pharynx had a length of 

 0.043 mm. and a width of 0.046 mm. and the intestine, which was wider 

 than the earlier stage and contained dark material, reached more than 

 one-half the body length. 



The great majority of rediae found were well advanced in devel- 

 opment and contained mature cercariae in their body cavities ( Fig. 37 ) . 

 The body cavity had increased in size extending from the birth-pore 

 almost to the posterior end, and into the posterior locomotor append- 

 ages. In each redia were germ balls and from two to four mature 

 cercariae, the bodies of which were from one-third to one-half the total 

 length of the redia. In a mature redia 0.81 mm. in length and 0.15 mm. 

 in width, the posterior locomotor appendages were 0.62 mm. from the 

 anterior end. The pharynx was 0.041 mm. long and 0.043 mm. wide 

 and the intestine which was somewhat distended with food material and 



