41 8 THE ORIGIN OF VERTEBRATES 



The Meaning ok 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 either 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 



