568 Cestodes from Selachians 
female organs; each testis is oval in shape, measuring 0-06—0-07 x- 
0-04—0-05 mm. 
The vas deferens (PL XXIII, fig. 11, L) is much coiled anteriorly to 
the cirrus pouch, the pouch being situated at about the anterior third 
of the segment. The duct is thin-walled and O-Ol —0-023 mm. in 
diameter; it enters the cirrus pouch, in which it is coiled many times 
and enlarges its diameter forming the cirrus distally (PI. XXIII, fig. 11). 
In the cirrus pouch the duct has a thick wall consisting of two muscular 
layers, the inner circular and the outer longitudinal. There are about 
50 longitudinal bundles of muscle fibres. The duct wall is lined with 
chitinous spinules and surrounded by distinctly nucleate cellular 
masses. 
The cirrus pouch (PI. XXIII, figs. 8 and 11,5) is very large, extending 
from its opening to the other side of the medullary field, it is ovoid 
shape and its wall is very thin; it opens into the common very thick- 
walled genital cloaca, which measures 0-17 x 0-08 mm. 
Female organs. These occupy mainly the posterior part of the 
segment, the vagina and uterus, however, run through the segment 
longitudinally. The ovary (PI. XXIII, figs. 8, 9,10 and 11, X) is situated 
posteriorly in the segment, it forms four irregular groups of elements 
extending dorsally and ventrally. Externally each group is continuous 
with the yolk gland and internally it protrudes into the segment, 
where the ovarian groups urtite midway to form an isthmus (figs. 9 
and 10, i); the latter communicates with the oviduct by means of the 
“egg-swallower” (figs. 9 and 10 E) which is situated in the middle 
of the isthmus, its wall being very muscular, it measures 0-04 mm. 
in diameter. 
The oviduct (PI. XXIII, figs. 10 and 11, 0) runs ventrally from the 
“egg-swallower” and soon unites with the proximal end of the vagina 
coming from the antero-dorsal side, the common duct then running 
dorsally to open into the shell gland (PI. XXIII, figs. 10 and 11, SD). 
The oviduct wall and proximal part of the vagina possess the same 
structure, the muscular wall being lined with cuticular cilia or 
spinules. 
The yolk gland (PI. XXIII, figs. 8,9 and 11, Z)) forms four longitudinal 
columns running forward from the posterior end of segment dorsally 
and ventrally to the lateral nerve cords, and lying between the marginal 
and medullary fields. Each column is of elongated oval shape in cross 
section (PI. XXIII, fig. 9, D). The shell gland (PI. XXIII, figs. 10 and 
11,>SZ)) is spherical or ovoid in shape and is situated a little postero- 
