14 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



toward the sides of the body and run forward nearly to its anterior end. 

 Here each brancli turns toward the median line and joins its mate dorsal 

 to the anterior portion of the pharynx. The excretory trunk and the 

 two branches which form this loop correspond, I believe, to the collect- 

 ing tubes or large excretory vessels of other trematodes. In sections 

 the wall of the vessel (Plate 2, Fig. 25, va. exc.) appears to be a structure- 

 less membrane, but occasional large oval nuclei are seen projecting 

 from the wall into the lumen of the vessel. In some sections I have 

 been able to make out very delicate circular and longitudinal fibres, and 

 from the peristaltic contractions of the vessel in living woi-ms it seems 

 probable that it is provided throughout its length with such fibres. 



I find a few small vessels arising from the lateral portions of the loop- 

 like vessel, but these could be followed only a short distance through the 

 parenchyma. Juel ('89) also found a number of small vessels, and 

 Pratt ('98) describes a pair of delicate branching longitudinal vessels in 

 livinor animals, though he was unable to make out their connection with 

 the larger vessels. 



There is considerable confusion in the interpretation of the parts of the 

 excretory sj-stem of appendiculate distomes. Olsson ('67) appears to be 

 the only one who has seen the excretory vessel, and he simply mentions 

 the presence of a vesicula caudalis in the tip of the tail of Distomum 

 [Hemiurus] appendiculatum. Juel ('89) states that he was unable to 

 find any excretory vesicle in the species that he studied and he suggests 

 that the vessel may itself occasionally become temporarily enlarged, 

 giving the appearance of a vesicle. He therefore believes an excretory 

 vesicle is wanting in appendiculate distomes and regards the entire 

 racket-shaped structure as excretory vessels or collecting tubes. 



Looss ('96, p. 123), in describing the excretory system of Apoblema 

 mollissimum, states his belief that the racket-shaped portion of the excre- 

 tory system represents the excretory vesicle. Pratt ('98) speaks of the 

 two branches which form the loop in Apoblema appendiculatum as if they 

 were a part of the vesicle proper (i. e. the excretory trunk), though he 

 believes it would be more in harmony with the custom of authors to 

 limit the application of the term vesicle to the unpaired portion of the 

 system and to call the branches and their anterior uniting ends the col- 

 lecting tubules. He adds that the entire structure forms a single organ, 

 and no part differs from any other part in structure or function. 

 Monticelli ('91) refers to the racket-shaped part as the trunk of the 

 excretory system, and Linton (: OO) speaks of it as the excretory 

 vessel. 



