360 H. s. PRATT, 



dix is invariably invaginated in these youngest worms and forms a ves- 

 icle with epithelial walls at the hinder end of the animal's body. It 

 is also extremely probable that shortly after the animal leaves the 

 Copepod, and sometimes before (PI. 26, Fig. 6), it sheds this epithe- 

 lium. The appendix begins to be evaginated after the first host has 

 been abandoned, and it is probable that if the epithelium is still pre- 

 sent when this happens, it quickly disappears as a result of the oper- 

 ation. When the worm is swallowed by its final host the appendix is 

 destitute of its epithelium and is covered by the cuticula alone. The 

 worm has never been observed in the final host, so far as I know, 

 with an epithelium covering the appendix. 



That no previous observer has seen and described this epithelium 

 is rather remarkable as the youthful appendiculate Distome has come 

 under the notice of a number of observers. Its occurrance has been 

 noted by Will (1844) in Beroë, Busch (1851) in Sagitta, Willemoes- 

 SuHM (1871) in Copepods,in worm-larvae, and also free-swimming, and by 

 GiESBRECHT (1882) iu Copepods. Yet none of these authors have 

 given a detailed description of the young worm, nor has Monticelli 

 (1891), who examined Giesbrecht's material, as already mentioned, 

 Monticelli, however, very accurately represents in his fig. 12 a young 

 Apoblema appendiculatum^ showing the extended appendix which is 

 covered with a membrane distinctly different from the cuticula of the 

 trunk. I have little doubt that this membrane was an epithelium 

 which, however, the author failed to recognise as such ; a failure which 

 was undoubtedly due to the poor preservation of the material, it hav- 

 ing been taken some fourteen years previously in the North Sea by 

 another person. 



It is also evident that the membrane at the back of this epithe- 

 lium, which is a continuation of the cuticula of the trunk of the worm, 

 becomes, when the epithelium has been cast, the external protecting 

 membrane of the appendix; it becomes its cuticula. 



We have, thus, a case where a membrane which bears an un- 

 doubted resemblance to a basement-membrane becomes the integ- 

 umentary covering or cuticula of the hinder portion of a Trematode. 



Various investigators have made similar observations on young 

 Trematodes (see Kerbert, 1881; Biehringer, 1884; Schwarze, 1886, 

 p. 50; Heckert, 1889; Looss, 1894). An epithelium, upon the outer 

 surface of which was a delicate cuticula, has been seen covering either 

 the entire outer surface of the young worm, or a portion of it: beneath 

 this, too, has always been seen the thick membrane which was destined 



