Life-history and Anatomy of the Appendiculate Distomes. 359 



which JuEL (1890) describes in the inner surface of the cuticula of 

 both trunk and appendix of the adult appendiculate Distome. The 

 fact that these vesicles appear in the inner surface of the entire 

 cuticula of the worm in Juel's preparations, and only in the outer 

 surface of the subepithelial membrane of the appendix of mine, in- 

 dicates that they have no morphological significance, but are patho- 

 logical structures, being exudations from the parenchyma in his specimens 

 and exudations from the epithelium in mine. 



The "intermediate layer" of Juel is also strongly developed in 

 the subepithelial membrane (PI. 26, Fig. 6) being the probable re- 

 sult, as explained above, of the strain of the retractor muscles, whose 

 insertions are in its inner surface. 



At the base of the extended appendix and at the point where the 

 epithelium begins, the outer layer of the cuticula of the trunk thickens 

 and forms a distinct ridge, which is raised to the level of the surface 

 of the epithelium. A deep circular groove is formed in this way 

 around the worm between the ridge and the anterior edge of the 

 epithelium (PI. 25, Figs. 3 and 4). 



I was so fortunate as to obtain one worm which was killed while 

 the integumentary epithelium of the appendix, above described, was 

 just being cast. The worm was still within the Copepod, with its 

 appendix retracted; the epithelium was entirely stripped off and lay 

 in a disintegrated mass in the appendicular vesicle (PL 26, Fig. 6 a^J. e). 

 The subepithelial membrane shows a roughened surface where the 

 epithelium has been torn from it. 



The worm above mentioned had a length of 0.5 mm: it was thus 

 of large size. All of the worms from whose appendices the epithelium 

 had disappeared were likewise of large size, and all of them were 

 taken free-swimming. In all of the smaller worms the epithelium was 

 still present and in some of those of larger size : the largest I observed 

 in which this was true is the one figured in Fig. 3, whose length, in- 

 clusive of the appendix which measured 0.13 mm, was 0.52 mm. 



It is evident from the above described observations that the appen- 

 dix of the youthful worm under discussion is, for a considerable time, 

 covered with a columnar epithelium, on the outer surface of which is 

 a cuticula. It is also extremely probable that this epithelium is pre- 

 sent on the appendix of the youngest worms when they emerge from 

 the cercarian stage of development in the body -cavity of their first 

 host, the Copepod: in other words, that the epithelium is a cercarian 

 structure which is retaiued for a while by the adult worm. The appen- 



