582 
Cestodes from Selachians 
area at the posterior portion of segment. The lateral halves of the 
ovary are connected by a comparatively narrow isthmus in the median 
plane, where the ovary is continuous with the ‘ egg-swallower” whose 
opening into the ovary is best seen in horizontal and sagittal sections. 
The ovum in the ovary measures 0-013 mm. in diameter and its nucleus 
(0-005 mm.) can easily be observed in haematoxylin-eosin preparations. 
The uterus (PI. XXIII, fig. 16, TJ) first makes its appearance trans¬ 
versely on the ventral side of the segment, and later its cavity enlarges 
more and more as it receives more ova; in gravid segments, it occupies 
all the available space, the other genital organs being atrophied. It is 
surrounded by a thin membrane, composed of cells with well-defined 
nuclei. It opens to the exterior a little anterior to the middle in the 
medio-ventral line of the segment (PI. XXIII, fig. 14). 
Male organs. The testes (PI. XXIII, fig. 16, H) are numerous and 
scattered chiefly anteriorly and dorsally to the female organs; they 
are oval or spherical in shape, measuring 0-11 x 0-05 mm. The vas 
deferens is thin-walled and coiled on itself, more so near the base of 
the cirrus pouch; it is enlarged by being filled with spermatozoa 
in the mature segments. On entering the pouch it widens (0-04— 
0-105 mm.) to form a vesicle-like duct and it is coiled two or more 
times inside the base of the pouch; the coiled part of the duct is thin- 
walled, but its distal end is straight, narrow (0-025 mm.) and has a thick 
muscular wall. The wall of the vas deferens in the pouch is surrounded 
by a single layer of clearly nucleated cells, like those in the vaginal 
wall (PI. XXIII, fig. 18, L). The protruded cirrus attains a length of 
0-4—0-6 mm. 
The cirrus pouch (PI. XXIII, fig. 18, B) is elongate pyriform, it 
measures 0-5 x 0-15 mm. basally, and is 0-09 mm. wide distally. Its wall 
is thin and surrounded by a layer of cells with distinct nuclei. The 
space between the vas deferens and the wall of the pouch is filled with 
parenchymatous tissue containing scattered nuclei. The cirrus pouch 
with the vagina passes dorsally to the excretory canals on the porose 
side. 
Affinities. There are some difficulties in deciding to which genus 
the worm belongs. The characters of the bothridium and the accessory 
sucker, the size, form and arrangement of the hooks offer important 
criteria for the systematic determination of Phyllacanthinae. Some 
differential characters of three genera are here tabulated (other genera 
of Phyllacanthinae may be omitted because of their being decidedly 
different). 
