264 R. T. YOUNG 



cannot be considered as definitely established. This uncertainty 

 is probably due in part at least to the difficulty of demonstrating 

 the excretory system in preserved material. Its delicacy renders 

 its tracing, at least to the finer branches, very difficult in pre- 

 served material. So far as I have been able to find the excretory 

 ducts in my own sections, I have seen no evidence of a definite 

 epithelium. Their walls apparently consist of a collection of 

 protoplasmic strands in direct continuity with those of the sur- 

 rounding parenchyma, in which are imbedded occasional nuclei 

 (fig. 5)._ _ 



Conditions in the reproductive ducts and glands show con- 

 siderable variation according to the statements of various authors. 

 In general, an epithelium is present, which is, however, very 

 variable in form. It may be insunken (penis of Byrsophlebs nana 

 von Graff, '03 and Geoplana pulla, von Graff, '91), and may lack 

 nuclei (penis of Yungia aurantiaca, Lang, I.e.), or both nuclei 

 and cell boundaries (Typhloplaninae, Luther, I.e.). The latter 

 author says (p. 98) : 



das Epithel des Atriums geht an der Spitze des Penis 

 in eine Kernhaltige Plasmamasse liber, in der sich keine Zellgrenzen 

 nachweisen lassen .... oft is sie (in the penis) nur noch 

 an den hier und da der Innenflache aniiegenden platten Kernen zu 

 erkennen, in anderen Fallen gelingt es iiberhaupt nicht mehr ihr Vor- 

 handensein festzustellen. 



In Plagiostomum reticulatum, von Graff ('08, p. 2287) de- 

 scribes the epithelium of the ''ausseren Penisrohrwandung" as 

 "vollends cuticulaahnlich" which ''farbt sich nicht mehr und 

 erscheint vollkommen homogen: im Ductus ejaculatorius prasen- 

 tirt es sich als eine haarscharfe, stark roth tingirte Linie." 



According to Luther (I.e.) in the Macrostomidae, an epithelium 

 is present in the antrum femininum alone, the gland ducts being 

 parenchyma spaces. The same author finds in the Mesostomidae 

 and most of the Typhloplaninae an epithelium in the virgin bursa 

 copulatrix, which later degenerates, leaving the basement mem- 

 brane superficial; the latter then becoming strongly developed. 

 In the bursa stalk, however, the epithelium persists. The inner 

 wall of the bursa seminalis of Gyratrix hermaphroditus is lined by 



