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THE AM ERIC AX NATURALIST [Vol. XLIII 



formation of the gonoducts as well as their structure, is 

 essentially similar to that of the body-wall. The walls of 

 each of these duets (except those of the uterus in ces- 

 todes) are formed at first of a single-layered epithelium 

 which develops from a primitive chord of epithelial cells 

 by the appearance in it of a lumen. This epithelium, 

 however, quickly degenerates and disappears and at the 

 same time the surrounding parenchyma secretes a cutic- 

 ula which forms the permanent coating of the tubes. The 

 spines which are often present in the cirrus and vagina 

 are formed in the same way. 



The terminal excretory vesicle has also primitively an 

 epithelial wall like that of the gonoducts, as has been 

 shown by Looss (1894) and in my study of Apoblema 

 (lSi)S), which is replaced by a parenchymatous cuticula 



The cause of the change in the structure of the walls of 

 these ducts from an epithelium to a parenchymatous 

 cuticula is pn.hahly identical with that which has been 

 brought about a similar change in the structure of the 

 body-wall. Not only is the outer covering of the animal 

 apt to be affected injuriously by the juices of the host, but 

 the walls of the large ducts opening to the outside as well, 

 and both have consequently undergone an identical trans- 

 formation. 



What then are the origin and function of the subcutic- 

 ular cells? That they belong genetically to the paren- 

 chyma has been proved with the utmost conclusiveness by 

 the embryological researches of Looss, Young, Balss and 

 others. The conclusion, based by Blochmann and Hein 

 upon anatomical evidence, that they form an epithelium 

 needs, but has not yet received, embryological support; 

 in fact, not a scintilla of embryological evidence has been 

 produced either by them or any one else in the thirteen 

 years which have elapsed since Blochmann's paper was 

 published. And purely anatomical evidence in an obscure 

 matter like this should be received with the greatest cau- 

 tion, especially since the extreme parasitism of these 



