STUDIES ON EARTHWORMS. 265 
be conveniently grouped, firstly by the presence or absence 
of “copulatory papille” at or near the genital pore, and 
then again grouped according to the number of their sperma- 
thecee. The number of setz per somite is perhaps rather 
variable in individuals of the same species—at any rate, they 
differ in the anterior and posterior somites. 
Evidence is continually accumulating for Lankester’s theory 
of the presence originally of two pairs of nephridia in each 
somite, and the modification of those of one series, in the genital 
somites, to serve as genital ducts. 
In Lumbricus, Titanus, Pontodrilus, the ventral series 
of nephridia persist, their apertures being in relation to the 
ventral sete. In these forms, then, the dorsal series of nephridia 
has disappeared, except in the genital region; that is, the 
oviducts are modified dorsal nephridia, the sperm ducts, and 
perhaps also the spermathece, are also dorsal nephridia which 
have shifted their position. 
In Rhinodrilus, Eudrilus, Anteus, Urocheta, Moni- 
ligaster [and Microcheta], the ventral series, except in the 
genital somites, have disappeared, the dorsal series remaining as 
nephridia. In Anteus it seems probable that a very slight 
modification of some of the dorsal nephridia in the clitellar 
regions enables them to perform the function of a sperm duct, 
as no distinct sperm duct is known. 
In one species of Acanthodrilus the dorsal and ventral 
series of nephridia are alternately suppressed (42), as Beddard 
has shown, whilst the same arrangement has been shown by 
Perrier to be present in Plutellus. 
So far as the oviduct is concerned, the homology is fairly 
obvious, since it opens internally in one somite, passes through 
the posterior septum and opens externally in the next, just as 
an ordinary nephridium does; and in Plutellus, where the 
nephridium does not pass through a septum, but lies wholly in 
one somite, so does the oviduct. 
The modification which the nephridium undergoes to form 
a genital duct consists either in— 
(a) A fusion of a series of nephridia; or 
