landek: anatomy of iiemiurus crenatus. 15 



Under these conditions I fail to see the harmony that exists in the 

 application of terms to this type of excretory system, and I do not find 

 a sino-le author who limits the term vesicle to the unpaired portion of 

 either the Y or racket-shaped excretory systems and designates the 

 branches as the collecting tubules. Braun (see Pagenstecher, A., und 

 Braun, M., '87-93) states that there are eases in which the branches of 

 the excretory vesicle pass gradually into the collecting tubes, so that 

 it is impossible to say just where one stops and the other begins. He 

 suggests that careful histological study may enable one to distinguish the 

 two parts, but for the present he believes that the best way to distinguish 

 them is by means of their contents. The vesicle contains larger or 

 smaller refractive granules, while the collecting tubes never contain 

 these, but are filled with a transparent or yellowish liquid which may 

 contain fine granules. Accordingly he considers the entire racket- 

 shaped portion of the excretory system to be a tube-like excretory 

 vesicle. 



As noted above, Looss ('96) and Pratt ('98) agree with Braun in 

 this respect, but, as already described, the posterior part of the unpaired 

 portion of the excretory system of Hemiurus ctenatus is differentiated 

 from the rest of the unpaired portion, and I believe this morphological 

 differentiation fully warrants me in considering it as the excretory 

 vesicle. The remainder of the racket-shaped portion I regard as the 

 collecting tubes, the excretory trunk or unpaired portion having prob- 

 ably arisen by a fusion of the posterior portions of the two tubes. 



Both Pratt and Juel found delicate muscle fibres on the wall of the 

 excretory vessel. Pratt could not make out any definite arrangement, 

 but Juel mentions two layers. 



Pratt, in comparing the appendicular vesicle, i. e. the unevaginated 

 appendix, with an excretory vesicle, speaks of its unusually thick walls 

 as though that feature might be regarded as an objection to considering 

 it an excretory vesicle, I have seen in young distomes, of a species 

 found encysted in the crayfish, a high columnar epithelium, the cells of 

 which are relatively much higher than those of the epithelium of the 

 appendicular vesicle as described by Pratt. The epithelium in the dis- 

 tome referred to is also shed, and the shedding takes jjlace at the time 

 the parasite enters its final host or shortly before. I am inclined to 

 believe that a similar process takes place in very many trematodes, since 

 the wall of the excretory vesicle is often described as a structureless 

 membrane without any traces of cells or nuclei. It is also interesting 

 to note that in the case cited the lining of the excretory vesicle was shed 



