278 
FRANK E. BEDDARD. 
nephridia of certain other Earthworms (e. g. Acan- 
thod rilus novse-zealandiae) have been derived 
through the gradual increase in calibre of the 
tubules forming the primitive network, which has 
become isolated into metamerically disposed tufts 
of tubules, corresponding more or less to the setse; 
these separate nephridia have become ultimately 
reduced to a pair in each segment. 
In the first case, therefore, the single pair of nephridia have 
been derived directly from a part of the primitive network ; 
in the second case the primitive network has become converted 
into a single pair of nephridia in each segment by a more 
gradual series of changes. 
The annexed scheme shows the relationship between certain 
genera of Earthworms, as indicated by their excretory system. 
Acanthodrilus 
Dichogaster 
Acanthodrilus multiporus 1 
This scheme, as will be seen, only refers to the genera which 
1 It is possible that this species is really a distinct generic type. It has no 
paired setse like the other species of the genus. If so, A. annectens 
(Beddard, 8) should probably be referred to the same genus. 
