46<î W. ^. N'KKERSON. 



(liaineter of the lumen of ihe tubule and they are separated by spaces 

 of from two to four times this unit. 



In many cases they appear to be attadied immediately beneath 

 nuclei situated in the walls of the tubules. 



The condition here described is that presented by the larj^^er and 

 more prominent portions of the tubules as seen in the anterior portion 

 of the body. Whether there is a <,M-adual transition from this condi- 

 tion to that of the short closely placed cilia of the hner capillary 

 tubules, I was not able to determine. 



The structure of these tubules is shown in PI. 31, Fijjf. 24 a— c; 

 a represents a portion of the left tubule shown in Vii^. 23 more hit,'hly 

 magnified. The thickness of the wall cannot be made out in the 

 whole prejiaration, ))ut the distribution of the nuclei of the cells which 

 f(»rm the walls is shown and several of the long cilia previously 

 described are discernable. In Fig. 24 6 and c the appearance of these 

 tul)ules is represented as seen in longitudinal and transverse section 

 respectively. I know of no criterion for determining whether the 

 lumen in this case is intercellular or intracellular. 



The condition of the excretory vesicle in Stichocofyle is 

 peculiar. The single median vesicle, which in most digenetic Trema- 

 todes lies near the posterior extremity of the body, has in this species 

 become divided into two symmetrical portions, so that there are two 

 separate excretory vesicles, one of which lies upon each side of the 

 intestine (PI. 29, Figs. 3 and 4; PI. 31, Fig. 23). 



These begin with their wider anterior ends opposite the pharynx 

 and extend backward nearly to the single dorsal excretory i)ore. 'i'hey 

 are nuich convoluted in the l)roader portions, the extent of the con- 

 volutions varying with the degree of contraction of the body, but 

 become straighter toward the narrowed posterior ends. Opj)osite the 

 bhnd end of tlie intestine they become suddenly reduced to very 

 narrow passages which lead inward to the median plane just behind 

 the end of the intestine, where they unite to form a single median 

 canal, which extends backward and ujtward to the dorsal excretory 

 pore (PI. 31, Figs. 22 and 23). That these two large lateral trunks, 

 which CuNNiN(;iiAM called lateral, excretory canals, are really the 

 homologues of the excretory vesicle, is a proposition which hardly 

 needs explanation or defence. A comparison of these structures with 

 the lateral trunks of Aspidognster leaves no room for doubt concerning 

 the moqthological equivalence of the two; in the latter form they 

 have been (»Itserved (VoKi/rzKow '88) to arise as simple enlargements 



