564 
Cestodes from Selachians 
neck (PI. XXIII, fig. 1) is narrow and long, measuring 0-5 X 0-17 mm. 
anteriorly where narrowest; it is marked with closely set transverse 
rings, giving its edge a serrated outline. 
The strobila is tenuous in the anterior part of body and gradually 
widens posteriorly, reaching its maximum breadth (about 1-1 mm.) 
at the posterior end (PI. XXIII, figs. 2 and 3). The segments rapidly 
increase in length toward the caudal end, the anterior segment measures 
0-12 X 0-12 mm.; the ripe posterior segment attains a length of 3—4 mm., 
being three or four times longer than broad. The worm’s surface 
is marked by closely set, transverse, slightly notched rings which 
give its margin a serrated appearance. This appearance, as noticed 
by Linton, can readily be seen with a low power, and constitutes a 
good specific character. The genital apertures are situated irregularly 
and alternately, opening at about the middle of the anterior half of 
the segment. 
Musculature ; there are two sets of longitudinal and transverse 
musculatures, situated as in other cestodes; be th sets are weakly 
developed. 
Male organs. The testes (PI. XXIII, figs. 4 and 5, H) occupy the 
anterior two-thirds or three-fourths of the medullary field,, they are 
oval in shape, measuring about 0-09—0*15 x 0-05—0-055 mm. Ante¬ 
riorly in each segment, the testes spread all through the medullary 
field, but towards the middle and the posterior segments they are 
displaced dorsally by the female organs. The cirrus pouch (PI. XXIII, 
figs. 4 and 5, B) opening into the common genital cloaca, is situated 
at right angles to body margin; it is oblong in shape, and measures 
0-36—0-43 X 0-17—0-25 mm.; in mature segments the pouch is usually 
distended by the seminal vesicle enormously enlarged with the sper¬ 
matozoa, and assumes an unusual form; the pouch wall is very thin 
and surrounded by a single cell-layer. 
The vas deferens (PI. XXIII, figs. 4 and 6, L) in the cirrus pouch is 
strongly convoluted. In young segments it is of uniform structure 
throughout its whole length, its wall being surrounded by a layer of 
cells with distinct nuclei and its diameter being nearly uniform (0-03 
mm.) excepting distally, where it enlarges to form the pyriform duct 
(0-07—0-10 X 0-045 — 0-05 mm.). In mature segments the vas deferens 
in the cirrus pouch exhibits two easily distinguished parts; (a) the 
distal part which is thick-walled, lined with minute retrograde spinose 
projections and surrounded by a thick layer of cells; {h) the proximal 
part which is thin-walled, its lumen being enormously distended with 
