276 
W. M. Wheeler 
secretoiy cliaracter of tlie tubules together with tbeir whole structure 
and arrangemeut leads me to regard tbem as true uephridia — 
and it is stränge tbat Nansen did not come to the same con- 
clusion b 
In the different species wbicb I have sectioned there is consider- 
able Variation in the opening of the nepbridia, both at tbeir posterior 
or cloacal nepbropores, and at tbeir anterior ends (nepbrostomes). 
Tbe two nepbridia may bave separate nepbrostomes, or may unite 
to form a single funnel opening into tbe body-cavity. Similarly tbe 
two nepbridia may nnite to form a single tube, or end-piece, wbicb 
opens into tbe cloaca, or tbey' may remain distinct and bave separate 
openings into tbe cloaca. Tbe conditions in eacb of tbe nine species 
are bere very briefly given: 
M. cirriferiim. Distinct nepbrostomes; a long unpaired 
end-piece opening into tbe anterior ventral end of tbe capacions 
cloaca (Fig. 5). 
M. circinatum. Distinct nepbrostomes; nepbropores 
paired, opening into tbe ventral portion of tbe cloaca. Tbis species 
agrees witb Nansen’s tigure oi M. giganteum ('85, PI. 1 Fig. 5). 
M. glahrum. Single nepbrostome; tbe nepbridia run for 
some distance side by side just ventral to the intestine, then fuse 
to form a long end-piece wbicb opens into the anterior ventral 
end of the capacions cloaca. In some specimens tbe secretion-granules 
are very irregulär and angular and stain very deeply. 
M. alatum. Single nepbrostome; unpaired end-piece 
as in M. glabrum. Tbe nepbridia contained immature and decom- 
posing ova, normal mature spermatozoa and mucous-like granulös. 
M. platypus. Single nepbrostome; separate nephro- 
pores opening into the cloaca rather far apart. Tbe cilia covering 
the nepbrostomes and extending along the ducts to tbeir openings 
into tbe cloaca are unusually long and distinct in tbis 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). 
2 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 all directed backwards, showing that the cur- 
