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no objection to the view here advocated, since several Annelids have 
only a single pair of these organs. Moreover the Myzostomes have 
in all probability undergone a very great reduction in the number 
of Segments, and we should, of course, expect a concomitant re- 
diiction in the number of pairs of nephridia. 
4, Thesegmental sacs (»suckers«). These prominent organs 
have not escaped the attention of any of the more careful students 
of the Myzostomidae, but hitherto all attempts at an Interpretation 
of their morphological significance have failed. Many authors have 
regarded them as suckers, but Observation of the living animal readily 
shows that they have no adhesive function, and the name is only 
another instance of the misleading nomenclature which has been 
employed in naming the organs of the Myzostomidae. 
The best and most recent description of the segmental sacs is 
given by Nansen ('85, pag. 42 — 44 and 76 — 77). I quote from his 
English resume (pag. 76): »What previous writers have called »suckers« 
are not really such, but ciliated glandulous sacks, as illustrated in 
PI. 8 Figs. 19 and 20. There are no muscular walls such as Geaff 
has described. The inner walls consist of a glandulous tissue, with 
large cells situated in one, or several layers; this tissue is covered 
by a ciliated cuticle, which is striated by the cilia penetrating into 
the tissue, vide Figs. 19 and 20. Only a few muscular fibres occur 
in the walls of the sacks, and those are usually dorso-ventral muscles 
which penetrate through the glandulous tissue and are secured to 
the cuticle by their extremities, vide Fig. 20 m. In M. glabrum^ the 
glandulous tissue is separated from the surrounding connective-tissue 
by a cuticle, on whose exterior side several muscles are situated, 
and I have not observed such a clearly defined cuticle in the other 
species examined. In the surrounding connective-tissue, vacuoli fre- 
quently occur. Among the glandulous cells, connective-tissue nuclei 
can be seen; this is especially the case in M. graffi and M. 
giganteum. — The openings of these sacks are, more or less, pro- 
minent in the various species, and the thickness of their oval margins 
also varies very much, vide PI. 8 Figs. 19 and 20. The form of the 
opening is in some species circular (e. g. M. cirriferum^ Fig. 22), and 
in others oval (e. g. M. giganteum^ Fig. 21). The openings are provided 
with sphincters, which vary in development in the various species; 
in some species they are few in number, and Ihick (e. g. M. cirri- 
ferum^ Fig. ‘lOsph)^ whilst in other species they are more numerous, 
but are then less developed in thickness (e. g. M. giganteum^ Fig. 19). 
