THE REGION OF THE SPINAL CORD 397 
organs in every segment directly derived from those of a polycheete 
ancestor, but also that such organs were partly somatic and partly 
appendicular in position. Such a suggestion is in strict accord with 
the observations of Sedgwick on the excretory organs of the most primi- 
tive arthropod known, viz. Peripatus, where also the excretory organs, 
which are true segmental organs, are partly somatic and partly 
appendicular. Further, the excretory organs of the Scorpion and 
Limulus group are again partly somatic and partly appendicular, 
receiving the name of coxal glands, because there is a ventral projec- 
tion of the gland into the coxa of the corresponding appendage. 
Judging from all the evidence available, it is probable that when 
the arthropod stock arose from the annelids, simultaneously with the 
formation of appendages, the segmental somatic nephric organs of 
the latter extended ventrally into the appendage, and thus formed 
a segmental set of excretory organs, which were partly somatic, 
partly appendicular in position, and might therefore be called coxal 
glands. 
As already stated, all investigators of the origin of the vertebrate 
excretory organs are unanimous in considering them to be derived 
from segmental organs of the annelid type. I naturally agree with 
them, but, in accordance with my theory, would substitute the words 
“primitive arthropod” for the word “annelid,” for all the evi- 
dence I have accumulated in the preceding chapters points directly 
to that conclusion. Further, the most primitive of the three sets 
of vertebrate segmental organs—the pronephros, mesonephros, and 
metanephros—is undoubtedly the pronephros ; consequently the 
pronephric tubules are those which I consider to be more directly 
derived from the coxal glands of the primitive arthropod ancestor. 
Such a derivation appears to me to afford an explanation of the diffi- 
culties connected with the origin of the pronephros and mesonephros 
respectively, which is more satisfactory than that given by the direct 
derivation from the annelid. 
The only living animal which we know of as at all approaching 
the most primitive arthropod type is, as pointed out by Korschelt and 
Heider, Peripatus ; and Peripatus, as is well known, possesses a true 
ceelom and true coelomic excretory organs in all the segments of the 
body. Sedgwick shows that at first a true ccelom, as typical as that 
of the annelids, is formed in each segment of the body, and that then 
this ccelom (which represents in the vertebrate van Wijhe’s pro-ccelom) 
