ON A PRORHYNCHID TURBELLARIAN. 641 



vitello-ovary. The latter (fig. 1, ov.) is long and narrow, 

 thrown into numerous sinuosities, and occupies a position on 

 the right side of the intestine corresponding exactly to that 

 occupied on the left by the testis, though it does not extend so 

 far back as the latter, stopping short some distance in front of 

 the posterior end of the intestine. It is enclosed in a fibrous 

 layer similar to and continuous with that enclosing the intes- 

 tine, and is lined internally with a layer of flattened granular 

 cells. Its interior is occupied, for the most part, by vitelline 

 cells or follicle cells. These are large cells arranged, for the 

 most part, in an epithelium-like manner, but sometimes col- 

 lected into more irregular clumps. Each contains a large 

 nucleus, and each has a distinct enclosing membrane. Through- 

 out the greater part of the length of the vitello-ovary the 

 follicle cells are loaded with large rounded yolk-granules, but 

 in the posterior portion these granules are entirely absent, and 

 the cells are much smaller. 



Here and there is an ovum (fig. 7). Each of these is a large 

 rounded cell with a large nucleus and very fine-grained proto- 

 plasm, enclosed in a follicle of regularly disposed follicle cells. 

 The ova are smaller and more numerous in the posterior part 

 of the ovary than they are in front, and here the follicle cells 

 that enclose each are numerous, and have the appearance of a 

 columnar epithelium. Further forward, where the ova are 

 large, the follicle cells that surround them are much smaller 

 than the rest. Towards the posterior end of the vitello-ovary 

 there is, in two of the three mature specimens, a great mass 

 of sperms distending the cavity, the wall of which is here very 

 thin; there is thus formed from a portion of the ovary a dis- 

 tinct, though perhaps temporary, bursa seminalis. Further 

 back again the vitello-ovary resumes its normal character. 

 In the sexually immature specimen the penis is completely 

 developed, but the various parts of the male duct have not 

 yet become formed. The testis is represented by a narrow 

 duct connecting together a chain of small cavities. In these 

 and in the lumen of the connecting duct are a number of cells 

 of a peculiar character, having the appearance of malformed 



