The Sexual Phases of Myzostoma. 
277 
M. helli. In this species 1 failed to find tlie nephrostomes ; 
there is an nnpaired end-piece, which does not open into 
the cloaca, but on the surface of the body through apa- 
pilla lying just ventral to the cloacal orifice. As the 
«Uterus« opens just above the cloaca, there are three ciliated apertures 
in this species, close together. In all three specimens the ne- 
phridia were full of mature spermatozoa evidently on 
their way to the exterior. 
M. cryptopodium. Single nephrostome; the ducts open on 
the surface by means ofan unpaired end-piece which perforates 
a papilla just under the cloacal orifice. 
M. eremita. As nearly as I was able to make out, the conditions 
in this species are like those in the two preceding. 
M. pulvinar. Distinct nephrostomes; the unpaired end- 
piece is short and narrow and opens on a distinct papilla which 
projects into the lumen of the cloaca. 
M. helli and cryptopodium tend to show that the nephridia 
originally opened on the surface of the body as in other Chaetopoda. 
In some species the unpaired end-piece has moved forward secondarily 
and acquired an opening into the cloaca. In still other species the 
nephropores have not fused, but have nevertheless undergone the saine 
shifting forward. This shifting is perhaps to be explained by the 
extent to which the ectoderm is infolded in the embryo to form the 
cloaca, but this question cannot be decided tili a careful study has 
been made of the alimentary tract and its relations to the insertions 
of the nephridia in as many species of Myzostoma as possible. The 
results of such a study could not fail to throw an interesting side- 
light on the obscure morphology of the Malpighian tubules in the 
Arthropoda. 
In M. helli the nephridia still retain the spermiducal function 
which they have in many Ch'aetopods. The position of the ne- 
phrostomes just back of the ovaries recalls the condition seen in 
many Chaetopoda; although the oviducal function has been transferred 
to the median unpaired opening of the body-cavity. 
The occurrence of but a single pair of nephridia is, of course. 
reut must pass through the tubules from the body-cavity to the exterior. Hence 
the view that the tubules may be respiratory in function has no foundation 
whatsoever. 
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