o'roke: larval trematodes. 165 



In the Ihing form these concretions look Uke highly refractive 

 cell nuclei and mark out the excretory tract very distinctly. The 

 excretory vesicle into which the paired tubes open is just an- 

 terior to the acetabulum and is .017 mm. in diameter. 



From the living specimen one would think that the tail con- 

 tains an excretory canal, but cross sections show that the central 

 region of the tail is made up of very large cells with extremely 

 delicate walls. Encircling this central region is a cylinder of 

 smaller cells and a narrow band of outer longitudinal muscle 

 fibers. 



The anlagen of the reproductive organs show as rounded masses 

 of deeply staining cells, within the diverticula of the digestive 

 tract. Cystogenous glands are present all over the body except- 

 ing in the immediate vicinity of the oral sucker and mouth. With- 

 in the cystogenous glands are granules of the cyst-forming ma- 

 terial. 



Of twenty-three specimens of Planorbis trivolvis collected at 

 Lawrence, Kan., October 10, one was infected with Cercaria dias- 

 trophora Cort. The following additional description and ac- 

 companying plates may be considered merely supplemental to 

 Cort's description of this form. These studies were made before 

 the identification of the species had been established as being the 

 same as C. diastrophora Cort. 



In general appearance this form corresponds to Cercaria cortii, 

 but it is much smaller, measuring .48 mm. long and .2 mm. wide. 

 The tail is .58 mm. long. The oral sucker is small, measuring 

 .06 mm. in width, while the acetabulum is exceedingly large, 

 measuring .15 mm. in outside diameter. 



The esophagus is very slender, opening into a bulbous pharynx 

 three-fourths as broad as the tip of the oral sucker. The eyespots 

 are prominent and are oval rather than spherical, measuring .027 

 mm. by .036 mm. They are separated by a distance equal to the 

 long axis of the eye. 



The excretory system consists of paired ducts opening into an 

 excretory vesicle anterior to the acetabulum. This vesicle also 

 receives smaller ducts from the region of the acetabulum. The 

 excretorj^ pore opens on the dorsal surface. 



This species stains deeply with hsemalum, excepting in the re- 

 gion of the oral sucker, where the cells seem to be devoid of nuclei. 

 The tail is made up of large central cells smToimded by a ring of 

 smaller cells. It contains no excretory canals. 



