If it is a right conception that the excretory organs of the protostracan 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 to excrete to the exterior? What should we look for in our search after the lost coxal glands?
The answer to these questions is most plainly given in the case of the pronephros, especially in Myxine, where Maas has been able to follow out the whole process of the conversion of nephric tubules into a tissue resembling that of a lymph-gland.
He states, in the first place, that the pronephros possesses a capillary network, which extends over the pronephric duct, while the tubules of the mesonephros possess not only this capillary network, equivalent to the capillaries over the convoluted tubules in the higher vertebrates, but also a true glomerulus, in that the nephric segmental arteriole forms a coil (Knauel), and pushes in the wall of the mesonephric tubule. He describes the pronephros of large adult individuals as consisting of—
1. Tubules with funnels which open into the pericardial cœlom.