386 
A Tri-Radiate Tapeworm 
is an internal vesicula seminalis in the cirrus sac separated by a short- 
narrow duct from an external vesicula seminalis outside the cirrus sac, 
the part of the cirrus sac through which this duct passes being much 
thickened. This corresponds with what occurs in the tri-radiate 
Cestode. On that hypothesis, the vesicula seminalis ( a.v.s ., p.v.s.) first 
mentioned is the internal one inside the cirrus sac, the second ( a.'v.'s 
p.'v.'s.') is the external one outside the cirrus sac, and the muscular 
organ (c.s.) is the only persistent, although abnormal, part of the cirrus 
sac through which the connecting duct between the two vesiculae 
seminalae passes. 
The course of the external vesicula seminalis (Fig. 3, a.’v.’s.', 
Fig. 6, p.’v.'s.') is very irregular. It always lies at the base of arm 1 or 
arm 3, and, after a more or less convoluted course, narrows to form a 
vas deferens (Fig. 3, a.v.d.), dividing and sub-dividing into numerous 
minute vasa efferentia. These intertwine with each other considerably, 
but finally lead to the small spherical testes. The latter lie in three 
separate groups in the parenchyma surrounding the central space, one 
group at the base of each of the three arms, extending along as far as, 
or occasionally beyond, the dorsal excretory vessel. The groups are not 
distinct from each other, but are only local aggregations upon a con¬ 
tinuous chain of testes surrounding the central space on three sides, 
the fourth and free side being that facing the ovary. 
The testes appear very quickly after the scolex and in quite young 
segments are well-developed: in those segments, vasa efferentia and 
vas deferens are also present. In older segments the testes commence 
to degenerate and in proportion as the male ducts develop tend to 
disappear. The spermatozoa are formed and are ejected into the vasa 
efferentia before the male ducts have completed their development, so 
that in young proglottides with testes the vas deferens is empty, but in 
older segments without testes it is filled with spermatozoa. As the 
male ducts are completed towards the central space, the spermatozoa 
move along them as far as the internal vesicula seminalis where they are 
usually stored. The development of the outer portions of the male 
ducts is at first apparently quite normal, each set being quite distinct 
from its neighbour: as development proceeds however, each set ap¬ 
proaches closer to its fellow, fusion eventually taking place when the 
internal vesicula seminalis is filled with spermatozoa ready for copulation. 
The common opening is then formed. This development accounts for 
the peculiarity before mentioned of the male ducts of a proglottis 
being always incomplete, possessing neither cirrus nor opening, when 
