172 BULLETIN OF THE 



In some specimens cross sections through the middle or anterior 

 reo-ion of the body show that the sac is moderately filled with cells 

 whose nuclei stand close together and are in a kinetic condition. Here 

 the ventral boundary of the dorsal line seems to be wanting in places, 

 as if the cells in the sac were directlj^ connected with those of the dorsal 

 line, while the wall of the sac is laterally directly continuous with the 

 basement membrane, which covers the line ventrally. One can some- 

 times find among the cells in this region stages which seem to show a 

 transition between the kinetic nuclei and the groups of spermatozoa 

 found among them ; but this condition was encountered in only a single 

 specimen, and the material was not in sufficiently perfect histological 

 condition to allow a study of spermatogenesis. This anterior portion of 

 the sac I regard therefore as testis, and the posterior crescent-shaped 

 portion as at once receptaculum and vas deferens. In the stage in which 

 the kinetic nuclei were found in the anterior portion of the sac, the walls 

 of the posterior portion were collapsed, and hardly a single spermatic 

 element was to be found in it. This is the youngest stage which I have 

 studied. 



In another, older stage the sac was filled from end to end with the 

 deeply stained highly refractive spermatozoa, and so enlarged that it oc- 

 cupied nearly the entire body cavity. Finally, in the oldest stage found 

 the sac (Fig. 11) appears in the anterior part of the body as a mere 

 remnant with collapsed walls containing an occasional spermatozoon. 

 At the tail, however, a small quantity of spermatozoa was collected near 

 the terminal orifice. The diminished thickness of the protoplasmic zone 

 in the body wall shows this individual to have been comparatively old. 

 In one specimen in which the sac was thus collapsed, however, the body 

 wall was moderately thick. In these cases copulation seems to have 

 taken place, and the few spermatozoa are merely remnants of the original 

 contents of the sac. 



The organ described by Biirger on pages 646, 647, is evidently the 

 same as this, and his belief that it was a testis rather than an ovary is 

 confirmed by the preceding account. The description he gives of the 

 organ either shows that the specimen studied by him was intermediate 

 between the first and second stages here described, or else was based 

 upon different individuals and represents different stages. 



In one of my individuals which, to judge from the thickness of the 

 body wall, must have been young, there were clusters of polyhedral cells 

 here and there in the anterior portion of the body cavity, and these 

 clusters were crowded full of spermatozoa in small bunches, as if they 



