STRUCTURE OF UROCHJ1TA AND DICHOGASTER. 277 
calibre of a part of the nephridial network in each 
segment to form a pair of nephridia, and by the 
gradual disappearance of the rest. 
The second way in which I conceive the gradual reduction 
of the network to a single pair of nephridia to have been 
brought about is as follows : 
The network became arranged metamerically by the isola- 
tion of the networks of successive segments at the septa ; at 
the same time the tubules themselves acquired a greater 
calibre. This stage is nearly reached in Deinodrilus, 
where the nephridial system forms a continuous series of tufts 
attached to the anterior wall of each segment ; but here and 
there in Deinodrilus the nephridia are connected through 
the septa with feebly-developed tufts of tubules lying on the 
posterior side of the segment in front. 
In Acanthodrilus multiporus this stage is exemplified ; 
all trace of the intercommunication between the nephridial 
systems of successive segments through the septa is lost, and 
the tubules are uniformly of greater calibre than those of 
Perichaeta; at the same time they are more decidedly re- 
lated to the setae of their segments. From this point the paired 
nephridia of other species of Earthworms have been derived 
either by a great increase in the calibre of the tubules coupled 
with the disappearance of part of the network and all the 
external orifices, except a pair to each segment (Dichogaster 
seems to be a stage further advanced than Acanthodrilus in 
the direction of those worms with a single pair of nephridia in 
each segment), 1 or by the breaking up of the network into 
separate nephridia. Brachydrilus (Bcnham, 10) offers an 
intermediate condition in this reduction ; the nephridial net- 
work has been broken up so as to form two separate pairs of 
nephridia in each segment. One pair then disappears, and 
the typical condition of the Earthworm excretory system is 
arrived at. 
I am disposed therefore to believe that the paired 
1 I have elsewhere (8) called attention to other points in which 
Deinodrilus is intermediate between Perichmta and Acanthodrilus. 
VOL. XXIX, PART 3. NEW SER. 
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