THE EPITHELIUM OF TURBELLARIA 



R. T. YOUNG 



From the Stale University of North Dakota 



SIX FIGURES 



The existence of an epithelium in trematodes and cestodes has 

 been a much debated question for many years, while among those 

 who deny the presence of this tissue in these worms there is much 

 difference of opinion as to the origin of this condition, some main- 

 taining that the epithelium has been lost, some that it has been 

 metamorphosed into the cuticula, while others fail to express an 

 opinion on this point. 



If we turn to the Turbellaria, the probable ancestors of tre- 

 matodes and cestodes, we find conditions which I believe give 

 a clue to the answer to this question. Most of these possess a 

 typical nucleated ephthelium, the cell boundaries of which how- 

 ever are difficult to observe without special methods; while in 

 the pharynx and its sack nuclei are often lacking in the epithelial 

 layer, having invaded the parenchyma to form an insunken epi- 

 thelium as has been experimentally observed by Jander ('97) in 

 the regenerating pharynx of Dendrocoelum. 



In many forms the general epithelium presents conditions simi- 

 lar to those common in the pharynx, while in some cases it has 

 been claimed that neither nuclein or cell boundaries are demon- 

 strable even with special methods of technique. Thus Bohmig 

 ('90) in several species of Alloeocoela was unable to demonstrate 

 cell boundaries in the epithelial layer by treatment with silver 

 nitrate, while the distribution of nuclei was very irregular. 



The typical conditions in the turbellarian epithelium are well 

 represented in Planocera inquilina (fig. 1). The surface of this 



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