418 THE ORIGIN OF VERTEBRATES 
THE MEANING OF THE DUCTLESS GLANDS. 
If it is a right conception that the excretory organs of the proto- 
stracan group, which gave origin to the vertebrates as well as to the 
crustaceans and arachnids, were of the nature of coxal glands, then it 
follows that such coxal glands must have existed originally on every 
segment, because they themselves were derived from the segmental 
organs of the annelids ; it is therefore worth while making an attempt 
to trace the fate of such segmental organs in the vertebrate as well 
as in the crustacean and arachnid. 
Such an attempt is possible, it seems to me, because there exists 
throughout the animal kingdom striking evidence that excretory 
organs which no longer excrete to the exterior do not disappear, but 
still perform excretory functions of a different character. Their cells 
still take up effete or injurious substances, and instead of excreting 
to the exterior, excrete into the blood, forming cither ductless 
glands of special character, or glands of the nature of lymphatic 
glands. 
The problem presented to us is as follows :— 
The excretory organs of both arthropods and vertebrates arose 
from those of annelids, and were therefore originally present in every 
segment of the body. In most arthropods and vertebrates they are 
present only in certain regions ; in the former case, as the coxal glands 
of the prosomatic or head-region ; in the latter, as the nephric glands 
of the metasomatic or trunk-region, and, in the case of Amphioxus, of 
the mesosomatic or branchial region. 
In the original arthropod, judging from Peripatus, they were 
present, as in the annelid, in all the segments of the body, and 
formed coxal glands. Therefore, in the ancestors of the living 
Crustacea and Arachnida, coxal glands must have existed in all the 
segments of the body, and we ought to be able to find the vestiges 
of them in the mesosomatic or branchial and metasomatic or 
abdominal regions of the body. 
Similarly, in the vertebrates, derived, as has been shown, not 
from the annelids, but from an arthropod stock, evidence of the 
previous existence of coxal glands ought to be manifested in the 
prosomatic or trigeminal region, in the mesosomatic or branchial 
region, as well as in the metasomatic or post-branchial region. 
How does an excretory organ change its character when it ceases 
