The Sexual Phases of Myzostoma. 
279 
Kound the openings radial fibres also occnr whose function is to 
act as dilatatores, vide Fig. 18 m, m'; Figs. 21 and 22.« 
Nansen is inclined to regard the segmental sacs as modified 
nephridia. The obvious objection that they have no communication 
with the body-cavity, he attempts to remove by referring to the 
antennal and shell-glands of Crustacea. He would regard the large 
cells forming the walls of the sac as glandular and believes that 
they secrete the mucous-like graniiles (vide his Fig. 19 PI. 8) which 
he finds in the cavity of the sac and among the cilia covering its 
ciiticnlar lining. He does not allude to a further objection to his 
view, viz. the obvionsly ectodermal origin of the segmental sacs. 
Although my own study of the segmental sacs is still far from 
being completed, I have thought it best to give in this connection a 
brief sketch of their occurrence, striicture, etc., reserving a fuller and 
more accurate account for future publication. The Organs in question 
occnr in nearly all, if not in all species of Myzostoma^ but the extent 
to which they may be developed, is highly variable. In general they 
may be said to be largest in the free-living species and reduced to 
mere rudiments or altogether absent (?) in the cysticolons and ento- 
parasitic forms. But that their reduction is not due exclusively to 
cysticolons habits is shown by M. platypus^ which has the largest 
and most elaborate segmental sacs hitherto observed in any Myzostome. 
On the other hand, in the active M. cirriferum the sacs are scarcely 
as well developed as in the sluggish species like M. glahrum and 
alatum. The extent to which the Organs may vary in structure will 
be seen from the following description of four ' species, which really 
present four distinct types of segmental sac: 
M, platypus. v. Graff who first described this species was 
impressed with the large size and odd appearance of its »suckers« 
and suggested that a careful Investigation might »yield some answer 
to the questions raised by Nansen in regard to the function and 
morphological import of these Organs«. Among my many sections 
I have found several which show the histological structure of the 
sacs fairly well — notwithstanding the rather poor and protracted 
preservation. 
Fach of the 8 sacs projects as a flat-topped circular tubercle 
above the general level of the ventro-lateral surface of the animahs 
body. The center of the tubercle is formed by a smoothly rounded 
boss, surrounded and partly enclosed by a thick sphincter-like ring, 
the surface of which is furrowed by two concentric circular grooves 
