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secretoiy character of the tubules together with their whole structure 
and arrangement leads me to regard them as true neph ridia — 
and it is stränge that Nansen did not come to the same con- 
clusion 1. 
In the ditferent species which I have sectioned there is consider- 
able Variation in the opening of the nephridia, both at their posterior 
or cloacal nephropores, and at their anterior ends (nephrostomes). 
The two nephridia may have separate nephrostomes, or may unite 
to form a single funnel opening into the body-cavity. Similarly the 
two nephridia may nnite to form a single tube, or end-piece, which 
opens into the cloaca, or they- may remain distinct and have separate 
openings into the cloaca. The conditions in each of the nine species 
are bere very briefly given: 
M. cirriferum. Distinct nephrostomes; a long unpaired 
end -piece opening into the anterior ventral end of the capacious 
cloaca (Fig. 5). 
M. circinatmn. Distinct nephrostomes; nephropores 
p a ir ed, opening into the ventral portion of the cloaca. This species 
agrees with Nansen's figure of M. giganteum ('85, PI. 1 Fig. 5). 
M. glabrum. Single nephrostome; the nephridia run for 
some distance side by side just ventral to the intestine, then fuse 
to form a long end-piece which opens into the anterior ventral 
end of the capacious cloaca. In some specimens the secretion-granules 
are very irregulär and angular and stain very deeply. 
M. alatum. Single nephrostome; unpaired end-piece 
as in M. glahrum. The nephridia contained immature and decom- 
posing ova, normal mature spermatozoa and mucous-like granules. 
M. platypiis. Single nephrostome; separate nephro- 
pores opening into the cloaca rather far apart. The cilia covering 
the nephrostomes and extending along the ducts to their openings 
into the cloaca are unusually long and distinct in this species 2. 
1 I had adopted this view long before the appearance of the paper in 
which it was announced by Beard ('94, pag. 403). 
- In some species cross-sections of the tubules show a cob-web like reti- 
culum extending across the lumina — an appearance also frequently seen in 
the mesonephric tubules of some Vertebrates (Amphibia). It is very difficult 
to decide whether this appearance is due to the presence of cilia glued together 
by the reagents or to the frayed out ends of the gland-like cells of the tubules. 
The cilia when distinct — and they are always very distinct in the nephro- 
stomes and nephropores — are ali directed backwards, showing that the cur- 
