122 ■'^- ooTo. 



this, however, I conoeive to be due not to the actii;il union of the yolk- 

 ducts, but to the circumstance that the paired portion of the vagina is 

 exceedingly short. The paired ducts of the two sides finally unite 

 with each other, in most species in the median line of the body; but in 

 Dididophora sessilis the point of union is displaced a little towards the 

 right side. In all the species the paired yolk-ducts form by their 

 union an enlargement, which during the period of reproductive activity 

 is usually swollen to a considerable size by the great quantity of 

 yolk-cells which it contains, and has received the name of vitelline 

 reservoir. In most genera this portion passes without any marked con- 

 striction into the contiguous parts; but in Epibclella and Tristomum it is 

 usually spherical in form and is distinctly set off from the other parts. 

 In Onchocotyle and Hexacotijle also, it passes without any distinct demar- 

 cation into the anterior part of the yolk-ducts, but is distinctly 

 set off from the unpaired yolk-ducts to which it gives rise (PI, XV, 

 fig. 1). The vitelline reservoir must not be regarded as a permanent 

 organ, being observable only when it is filled with yolk-cells. 



From the yolk-reservoir proceeds the unpaired yolk -duct. This 

 is usually much smaller than the terminal portions of the paired ducts, 

 and is therefore distinctly marked off from the reservoir. In most 

 species it either takes its rise on the ventral side of the reservoir or 

 proceeds from it directly backwards, and after a course of various 

 length according to the species finally opens into the oviduct. In 

 Hexacotijle, however, it proceeds laterally from the reservoir and unites 

 with the oviduct. In Galicotijle the unpaired yolk-duct is exceedingly 

 short, and in Monocotyle it is entirely wanting, the paired ducts open- 

 ing in this genus separately into the oviduct from either side. 



Where the various genital ducts come to lie in the same 

 sagittal plane, the yolk-duct is generally situated ventrally to them, 

 but in Calicotijle, in which it opens into the seminal receptacle. 



