94 Transactions of the Royal Canadian Institute. [vol. x 



vessels open without any change of diameter. The junction of the 

 median vessel and the vesicle is, on the other hand, not abrupt but 

 gradual or funnel-shaped. In the scolex all three vessels take a compara- 

 tively straight course, gradually narrowing until, as they enter the first 

 proglottis, they are, median, 30/x, and lateral, 8/x in diameter; while 

 in the anterior proglottides they take the form of irregular spirals, the coils 

 of the lateral vessels following more or less those of the median, ex- 

 cepting as they pass the interproglottidal space where they narrow down 

 and straighten out slightly. The comparatively small size and straighter 

 course of the vessels in the scolex is doubtless due to the great develop- 

 ment of the dorso-ventral bothrial muscles through which they pass. 



Posteriorly their course is modified by the development of the re- 

 productive ducts in the median line. This applies more particularly to 

 the larger median vessel, since the other two, being situated ventro- 

 laterally, are not much disturbed. Between the sets of reproductive 

 ducts the median vessel lies in the median coronal plane, separating the 

 testes into two lateral fields (Figs. 17 and 18), while the smaller vessels 

 are situated below the testes but within the ring of vitelline follicles. 

 As the former approaches the cirrus-sac it usually rises (it is somewhat 

 depressed in Fig. 38) and passes dorsally to the right or left, along the 

 uterus-sac and over the ovary and lateral portion of the generative 

 space to the median line again. However, it frequently crosses from 

 one side to the other dorsal to the anterior end of the developing uterus- 

 sac or the space between it and the opening of the vagina, as shown in 

 Fig. 17. But the greatest changes in the course of these vessels comes 

 when the uterus becomes gorged with eggs. The smaller vessels then 

 appear greatly flattened laterally, within the testes that appear in these 

 sections, and not so distinctly towards the ventral surface (Fig. 19). No 

 trace of the larger median vessel is to be seen along the middle of the 

 uterus-sac excepting in younger stages where it is in the form of an 

 almost obliterated tube situated dorso-laterally. Anteriorly and pos- 

 teriorly, however, in several series this vessel apparently passed right into 

 the uterine sac tangentially, the opening thus being closed with a flap- 

 like valve. While this was very difficult to make out and was considered 

 of only secondary importance, it was thought that perhaps the much dis- 

 tended condition of many of the uteri in the posterior end of the strobila, 

 especially behind the region of closure of the temporary uterine opening 

 (vide infra) , might be due to fluid from the median vessel escaping into 

 the uterine cavity by the absorption or rupture, during distension, of the 

 two extremely thin walls between. 



The relations of the excretory vessels in what has been called the end- 

 proglottis are rather peculiar. The median vessel (Fig. 12) gradually 



