The Histogenesis of Cysticercixs pisiformis. 227 



that, as I have already shown, a part of the ciiticula may he 

 cast off during development, but any indication of a cellular 

 structure therein is entirely lacking. I therefore narrow this dis- 

 cussion to the question as to whether the sub-cuticula is simply 

 a modified parenchyma layer, or a true epithelium. The following 

 discussion will apply equally to the sub-cuticula and the so-called 

 epithelium of the excretory ducts, since the structure of the outer 

 layer of the Cestodes is essentially similar to that of the wall of 

 the ducts. In both one finds a cuticula, muscle and parenchyma 

 fibres, and sub-cuticular cells which possess a more or less definite 

 epithelial arrangement. The structure of the wall of the reproductive 

 glands and ducts will not be considered in detail, since these organs 

 have not been studied.^) The arguments which have been advanced in 

 favor of the epithelium theory may be briefly summarized as follows : 



1. The sub-cuticular cells are sharply differentiated from the 

 underlying parenchyma and are not in union therewith. 



2. The sub-cuticular cells are the matrix cells of the cuticula, 

 into which their processes extend. 



3. The presence of sense cells in the sub-cuticula, which are 

 to be considered as epithelial derivatives. 



4. The presence of gland cells in both Cestodes and Trematodes. 



5. The sub-cuticular cells are the absorptive cells. 



6. The flame cells, which are gland cells and hence epithelial, 

 originate primarily in cells lining the excretory ducts. Hence the 

 ducts are lined by an epithelium. 



The objections urged against the epithelium theorj^ are: 



1. The sub-cuticular cells are in intimate union with the paren- 

 chj^ma cells, and in some cases, as in the bladder wall of Cysticercus, 

 are strictly identical with the latter in form. 



2. The}^ are deeply imbedded in the parenchyma and often 

 widely separated from each other by it; thus they do not form the 

 outer layer of the Cestode bodj^, but are separated from the cuticula, 

 with the exception of the fine processes which they send into the 

 latter, by a parenchyma layer — the basement membrane, and a 

 layer of muscles. 



3. The outer epithelium is ordinarily conceived as an ectodermal 

 derivative. But the ectoderm in Cestodes is considered to be cast 

 off in the course of development. Therefore the Cestodes cannot 

 possess an epithelium. 



1) See Appendix. 



