The Sexual Phases of Myzostoma. 
281 
of the sac as a kind of gland, but granules of exactly the same 
kind are deposited, often in conspicuous masses, in the oiiter paren- 
chyma of the sphincter-like ring (Fig. 29), in the cordate elevations 
at the bases of the parapodia and in other portions of the animars 
body. This, together with the fa et that these granules seem never 
to be exereted from the body, leads me to suspect that they are 
pigment granules. 
M, glahrum. In sections the retracted sacs of this species ap- 
pear as spherical or pear-shaped bodies with walls so thick that the 
eavity is obliterated or redueed to an irregulär ramifying slit be- 
tween the folds of the walls. The walls themselves consist mainly 
of large succulent cells which undoubtedly correspond to the large 
gland-like cells of M. platypiis and which in some iron-haematoxylin 
specimens are full of very fine black granules. These granules, how- 
ever, resemble densely aggregated cytomicrosomes rather than the 
secretion- granules of gland- cells. Peripherally the succulent cells 
are covered with a dense striated cuticle, which probably bears 
rigid cilia in the living Myzostome. To this cuticle are attached the 
radiai muscle fibres that run between the large succulent cells and 
function as retractors of the walls of the sac. In sections stained 
with iron-haematoxylin these fibres are often color ed black, while 
the intervening cells stain much more faintly. When the sac is 
everted it forms a somewhat irregularly folded fungiform elevation 
on the surface of the body. 
M. cirriferum. In this species the sacs are flattened and have 
a distinct cavity and much thinner walls than in the preceding species 
(vide Nansen's Fig. 20 PI. 8). The rigid cilia covering the cuticular 
lining of the sac are very distinct, while the large cells of the walls 
are much vacuolated. These vacuoles are probably produced by the 
sudden withdrawal of the water from the succulent cells during 
preservation. The retractor and sphincter are correctly represented 
by Nansen. 
M. pulvinar. . In this species the sacs were said to be absent 
by V. Graff ('84 b), but Prouho has succeeded in detecting them 
('92). In hardened specimens they are readily found as small white 
spots not far from the reflected edge of the body in the dorsal 
(morphologically ventral) surface (PI. 12 Fig. 41). In sections each 
spot is seen to correspond to a small pit or dimple which is covered 
with rigid cilia. Beneath the dimple the parenchyma or, more 
probably what corresponds to the remains of the large succulent 
