No. 516] THE CUTICULA OF CESTODES 



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appears as a fine line between the ninscle-layers and the 

 outer epithelium (ectoderm) of the redia and eercaria, 

 which gradually broadens and when the epithelium is 

 finally shed, becomes the outer covering of the body. 

 Looss has observed this proceeding in a dozen or more 

 different species of trematodes and believes it to be gen- 

 eral to the entire group. 



According to Young, the cuticula of the young cysti- 

 cercus is a delicate layer which is composed of a ground- 

 work formed of fibrillar projections of the embryonic 

 parenchyma cells and a homogeneous translucent cement- 

 like substance produced by these cells (Figs. 8 and 11). 

 The subcuticular cells have not yet differentiated from 

 the embryonic parenchyma. The young cuticula soon be- 

 gin to scale off on the outer surface and is constantly 

 being added to on the inner. In the course of time two 

 layers show themselves in the cuticula, the inner of which 

 alone contains the cement-like substance, the outer layer, 

 which in later stages may be very thin or be entirely lost, 

 forming the so-called hair-layer which sometimes char- 

 acterizes the outer surface of cestodes and is exclusively 

 fibrillar. 



The later history of the cuticula is a continuation of its 

 larval history. Although formed principally as a secre- 

 tion of the parenchyma, it is at all times a part of it and 

 will often show, even in the adult stage, a fundamentally 

 fibrillar structure. It is also, as has been stated, never 

 moulted and can not be easily separated from the struc- 

 tures beneath. 



Very important in a study of the whole question, is the 

 relation of the gonoducts and the excretory vesicle to the 

 surrounding parenchyma, inasmuch as the walls of these 

 structures have essentially the same structure as the 

 outer body-wall, being lined by a cuticula at the back of 

 which are the parenchyma and usually muscle fibers. 

 What then is the developmental history of these ducts? 

 It has recently been shown by Balss (1908) in cestodes 

 and Boewer (1906) in trematodes that the history of the 



