266 R. T. YOUNG 



Kriechleiste nur einen socundaren Charakter erkennen — einen Charakter, 

 der erst in den beiden am weitesten differenzirtcn Familien der Rhyn- 

 codemidae und Bipaliidae auftritt und bei letzterer seinen hochsten 

 Ausliildungso-rad erroiclit hat. Hicr ergroift der Process der Einsenkung 

 bei manchcu Formen, wie Pkic. kewensis, das gesamte Korperepithel 

 und man konnte sagen dass diese Species und die ihr im Baue des Epi- 

 thels zunachststehenden Bipaliiden im Begriffe sind, die Epithelform der 

 Trematoden und Cestoden zu acquiriren. 



My own observations emphasize this view of von Graff, which 

 has not yet received sufficient notice. I must, however, differ 

 from him in assigning an epithehum to cestodes and trematodes.^^ 

 My own view is that we have progressing in the Turbellaria an 

 epithehal transformation leading to the condition in the former 

 groups in which the epithelium has been replaced by a cuticula. 

 I find this process occurring in ontogeny in the vagina and penis 

 of Taenia serrata, as I hope to explain more fully in a forthcoming 

 paper. 12 



A similar process has been described by Lonnberg ('91) in the 

 vagina of Abothrium rugosum and Tetrarhynchus tetrabothrius; 

 he has also pointed out the probable homology between the 

 turbellarian epithelium and the cestode cuticula. ^^ 



The presence of nuclei in the cuticula of the primitive cestode 

 Amphilina (Salensky, '74) and in several trematodes (Mono- 

 stomum mutabile — Braun, '93; Distomum sp. — Maclaren, '05; 

 Cotylogaster — Nickerson, '02, etc.) suggests that these are forms 

 in which the outer layer has not yet been fully evolved into the 

 cuticula typical of these worms. 



The many observations among both trematodes and cestodes 

 of the sloughing of the larval epithelium, this being later replaced 

 by a cuticle formed from underlying tissues (Leuckart, '86, 

 Looss, '92, '93, '94, Pratt, '98), etc., does not, T beheve, detract 

 from the soundness of this view, because the homology of the 

 larval epithelium is by no means certain. Until more is known 

 concerning the germ layers of plathelminths, speculation on this 

 latter point is futile. 



" See my discussion of this question elsewhere (Young ('08). 

 "'Here however there is apparently no insinking of nuclei into the paren- 

 chyma. 



"See also Ziegler ('05). 



