26 ILLINOIS BIOLOGICAL MONOGRAPHS [306 



membranous sheet and which have large, rounded, often vacuolated bodies 

 extending irregularly into the canal. The protoplasm of the cells is 

 granular. 



Excretory System. — In this family as in all Heterocotylea, there are 

 two excretory pores, situated on the dorsal surface about midway between 

 the median line of the body and the lateral edge of the worm, near the 

 level of the caudal margin of the pharynx (Fig. 27, 33). These open 

 from vesicular expansions, which when filled are almost spherical and 

 when empty have folded walls. The descending collecting duct originates 

 in the region of the pharynx from the fusion of smaller ducts and passes 

 posteriad to the region of the caudal disc where it turns cephalad and 

 continues as the ascending collecting duct to open into the excretory 

 vesicle. Both the descending and ascending ducts receive smaller 

 branches at irregular intervals; at the caudal end of the body a canal 

 joins the tubes of the two sides and a similar connection exists between 

 the descending ducts just anterior to the pharynx. From this anterior 

 communicating canal a branch enters the anterior sucker near the median 

 line. The excretory vesicles are lined with a thin layer of cutieula con- 

 tinuous with that of the external surface of the body and the collecting 

 ducts and accessory branches have a fibro-membranous wall in which 

 nuclei are occasionally embedded. In P. integerrimum, Zeller described 

 many connections of the collecting ducts of the two sides thru anastoinoes 

 of their smaller branches. He also described cilia on the walls of the col- 

 lecting ducts. Looss (1885) described the excretory system of P. ocella- 

 tum. He says the collecting ducts are not ciliated throuout, but only in 

 occasional areas, and describes cilia in the capillaries. These capillaries 

 are long and at the distal end are very much coiled. In this coiled part 

 the capillary is divided so that two flame cells discharge into each coil 

 and are emptied by a single capillary. The caliber of the excretory ves- 

 sels is very minute, and altho varying somewhat as a result of distention, 

 lacunar expansions were not observed. Because of the limited amount of 

 material, much of which was received in a preserved condition, no at- 

 tempt was made to trace the excretory system in living worms of this 

 family. The vitellaria completely obscure the excretory ducts in toto 

 preparations. The secondary ducts are so small and so often collapsed 

 that it is impossible to follow their continuity with certainty in sections. 



Nervous System. — The morphology of the nervous system of P. 

 integerrimum was described in detail by Andre (1910). He described a 

 supra-esophageal brain from which three pairs of nerves pass anteriad 

 and three pairs posteriad. In another paper (1910a) he gave a detailed 

 description of the eyes of P. integerrimum. In the present work no 

 special study of the nervous system was made and no new facts were 

 adduced. 



