EPITHELIUM OF TURBELLARIA 265 



a plasma layer continuous with "die Vacuolisirte Ausfiillungsmasse 

 . . . . in welcher zerstreute Kerne neben Spermamassen 

 eingebettet sind" (von Graff, '05, p. 140). A similar condition is 

 presented by the bursa copulatrix of Monoophorum durum (Fuhr- 

 mann. I.e.)- Until more information is available regarding the ori- 

 gin of ovary, testis and yolk glands in Turbellaria, speculation 

 concerning the presence or absence of an epithelium in these or- 

 gans had better be postponed. At present, statements of different 

 authors are divergent, and in many cases not specific, on this point, 

 some, as Lang (I.e.), claiming its existence, while this is denied or 

 not mentioned by others. 



In some places (pharyngeal sack, seminal vesicle), the wall 

 of the organ may be sufficiently distended to cause the disappear- 

 ance of the epithelium in places so that the underlying muscles 

 abut directly on the lumen (fig. 6) . Similar conditions have been 

 described by Lang (I.e.) in the penis of Yungia aurantiaca, Luther 

 (I.e.), in the atrium copulatorium of Castrada segne, the bursa 

 copulatrix of Mesostomidae, etc. 



We thus find in the Turbellaria a complete series of stages in 

 the modification of a normal epithelium to one with insunken 

 nuclei and cell boundaries, not readily demonstrable except by 

 special methods of technique, ^^ and similar transition stages 

 occasionally occur in the same species as pointed out by von Graff 

 ('91, p, 6) in the following words: 



bei ihnen (Convoluta sordida and paradoxa) ein und 

 dieselbe Hartungs- und Farbungsmethode (Hamatoxylin z. B.) bald 

 zahlreiche Epithelkerne hervorhebt, bald gar keine oder doch nur 

 sehr wenige Kerne des Epithels tingirt, wenn auch in alien iibrigen 

 Theilen bei beiden Individuen die Tinktion eine gleich tadellose ware. 



Von Graff ('99) has called attention to this transition, point- 

 ing out the resemblance between the insunken epithelium of 

 Turbellaria and the 'epithelium' of trematodes and cestodes. 

 He says (I.e., p. 42) : 



Beriicksichtigt man dass in der urspriinglichsten Familie (Geoplani- 

 dae) .... ein normales Epithel angetroffen wird, so kann 

 man in den eingesenkten Epithelien liberhaupt und speciell in dem der 



'■" Not even then, in all cases, fide Bohmig ('90). 



