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FEANK E. BEDDAED. 
ments and their apparently different function from the nephridia 
in other parts of the body (if one may so interpret M. Perrier's 
experiments), renders plausible a comparison of this part of the 
excretory system with the “ mucous gland” of Urochseta. 
If this comparison be allowed the most important consequences 
follow; it would seem, in fact, as if the specialisation of this 
part of the nephridial system ultimately led to the concentration 
of the numerous excretory pores into one long duct; that 
in fact the branched mucous gland of Urochseta is 
traceable to the specialised nephridial mass of the 
anterior segments of Perichseta; the numerous ex- 
ternal pores of the latter being replaced by the single 
aperture of Urochseta. 
I have in a previously published paper pointed out that if 
the peculiar cutaneous glands of Urochseta correspond to 
abortive setse, as they appear to do from a comparison with 
similar glands in Anachseta (Yejdovsky, 29, pi. vii, fig. 1), 
the eight setse per segment of Urochseta are brought about 
by a reduction of a complete circle of setse such as exists in 
Perichseta. On this hypothesis Perichseta is the primitive 
form, Urochseta comes next, and finally Dichogaster and 
Acanthodrilus, in which there is no trace of the missing setse, 
complete the series. It will be noticed that the evolution of 
the mucous gland, as I have traced it in the foregoing pages, is in 
correspondence with this series of facts. 
Nephridia. — All the segments of the body in Urochseta 
from the fifth are furnished with a pair of nephridia. 
The external apertures of these are perfectly plain on the ex- 
terior of the body. 
Perrier has already referred to the fact that the aperture of 
the nephridium is surrounded by a peculiar cup-like structure, 
which seems to be composed of radially arranged, short muscular 
fibres. He has also figured the funnel. I find that with 
respect to the funnel there is a remarkable difference between 
the mucous gland and the nephridia of the anterior segments 
on the one hand and the posterior nephridia. Perrier's 
figure of the nephridial fuunel (22, pi. xvi, fig. 42) evi- 
