The Sexual Phases of Myzostoma. 
279 
Round the openings radiai fìbres also occur whose fimction is to 
act as dilatatores, vide Fig. ISm, m\ Figs. 21 and 22.« 
Nansen is inclined to regard the segmentai sacs as modified 
nephridia. The obvioiis objection that they bave no commimication 
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 glandulär and believes that 
they secrete the mucous-like granules (vide bis Fig. 19 PI. 8) which 
he finds in the cavity of the sac and among the cilia covering its 
cuticular lining. He does not allude to a further objection to his 
view, viz. the obviously ectodermal origin of the segmentai sacs. 
Although my own study of the segmentai sacs is stili far from 
being completed; I bave thought it best to give in this connection a 
brief sketch of their occurrence, structure, etc., reserving a fuller and 
more accurate account for future publication. The organs in question 
occur in nearly ali, if not in ali 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 cysticolous and ento- 
parasitic forms. But that their reduction is not due exclusively to 
cysticolous habits is shown by M. platypus^ which has the largest 
and most elaborate segmentai sacs hitherto observedin anyMyzostome. 
On the other band, 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 segmentai 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«. Amoug my many sections 
I bave found several which show the histological structure of the 
sacs fairly well — notwithstanding the rather poor and protracted 
preservation. 
Each of the 8 sacs projects as a flat-topped circular tuberete 
above the general level of the ventro-lateral surface of the animaFs 
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 
