THE REGION OF THE SPINAL CORD 403 
along the whole length of the body, and were supplanted by the 
mesonephric tubules, which also belonged to the same segments. 
I also think that the paired appendages which have left the pro- 
nephric tubules as signs of their past existence, existed originally, in 
the invertebrate stage, on every segment of the body. But I do not 
consider that such a statement is at all equivalent to saying that such 
pairs of tubules must have existed upon every one of the segments 
existing at the present day ; for it seems to me that Riickert is much 
more likely to be right when he says that in Selachians the duct 
clearly does grow back, and is not formed throughout in situ ; so that 
he gives a double explanation of the formation of the duct—a palin- 
genetic anterior part formed by the fusion of the extremities of the 
original excretory tubules, to which a posterior ecenogenetic lengthen- 
ing has been added. 
It does not seem to me at all necessary that the immediate inver- 
tebrate ancestor of the vertebrate should have possessed excretory 
organs which opened out separately to the exterior on each segment ; 
a fusion may already have taken place in the invertebrate stage, and 
so a single duct have been acquired for a number of organs. Such a 
suggestion has been made by Rickert, because of the fact discovered 
by Cunningham and E. Meyer, that the segmental organs of Lanice 
conchilega are on each side connected together by a single strong 
longitudinal canal. I would, however, go further than this and say, 
that even although the nephric organs of the polychete ancestor 
opened out on every segment, and although the primitive arthropodan 
ancestor derived from such polychzete possessed coxal glands which 
opened out either on to or at the base of each appendage, similarly to 
those of Peripatus, yet the immediate arthropodan ancestor, with its 
paleostracan affinities, may already have possessed metasomatic coxal 
glands, all of which opened into a-single duct, with a single opening - 
to the exterior. 
Judging from Limulus, such was very probably the case, for 
Patten and Hazen have shown (1) that the coxal glands of Limulus 
are segmental organs belonging to the prosomatic segments; (2) that 
the organs belonging to the cheliceral and ectognathal segments 
are not developed; (3) that the four glands belonging to the endo- 
gnaths become connected together by a stolon, which communicates 
with a single nephric duct, opening to the exterior on the basal 
segment of the 5th prosomatic appendage (the last endognath). At 
