44 ON THE ANATOMY OF MEGASCOLIDES AUSTRALIS, 
number, and the network undergoes corresponding decrease with, it may be, as in 
Megascolides, the formation of a single duct on each side of the segment in connection 
with a network in the parts where the small nephridia still persist. 
(7) The final disappearance of all trace of the small nephridia, and with them of 
the network and longitudinal duct. There then remains in each segment, as in most 
adult earth-worms, a limited actor ate one pair—of large nephridia, with 
internal and external openings. 
Connection of Nephridial Organs and Genital Ducts.—LanxustER was the first 
to suggest a probable connection between the two structures in earth-worms, and 
also to suggest that in the earth-worm two sets of nephridia had primitively been 
developed,* of which one now persists as the nephridial organs; the other only 
remains in the segments bearing the genital organs, and has become modified into 
genital ducts, the nephridia of this system having hence disappeared everywhere save 
in the genital segments. These two sets of nephridia were supposed to be related to 
the two pairs of setee—the persistent nephridia to the ventral, and the nephridia now 
persisting as genital ducts to the dorsal pair of sete. 
Subsequent investigations have shown that he was perfectly right in supposing 
that primitively there was more than one pair of nephridia to each segment, but it is 
now very doubtful indeed whether there is much evidence in favour of his theory 
that the genital ducts are to be regarded as modified nephridia in the Terricole. 
The evidence derived both from the structure of the two sets of organs, and from 
a consideration of the state of development of the respective structures in different 
earth-worms is opposed to the truth of his view. . 
First, with regard to the structure, which is very suggestive, though not 
perhaps of so great importance as the second class of evidence. The general structure 
of a highly developed (but not primitive) nephridium in an earth-worm is that of a 
tube with a funnel-shaped opening into the ccelom at one end, and an opening on to 
the surface of the body at the other extremity. In some part of the funnel-shaped 
structure the duct is inter-cellular, but the funnel, apparently without exception, leads 
back into an wntra-cellular duct, always of considerable length and complication, and 
it is this «intra-cellular duct which forms the most characteristic portion of a 
nephridium, and which is never absent, however slightly the wter-cellular portion 
may be developed. The more primitive the nephridium (as in Acanthodrilus, 
Megascolides, &¢.,) the less developed becomes the inter-cellular part, which forms a 
duct leading from the intra-cellular part to the surface, and in connection with 
which—but only in the most highly developed forms—a vesicle may be developed. 
On the other hand, the whole of the genital duct ws inter-cellular, so that of the vasa 
deferentia or oviducts correspond to nephridia, all the intra-cellular portion of the 
duct between the funnel and the inter-cellular duct opening externally must have been 
entirely lost. 
* Q.J.M.S., 1865, p. 18. On the Anatomy of LZumbricus. 
