428 THE ORIGIN OF VERTEBRATES 
might be expected to help in the elucidation of the origin of the 
thymus. 
The origin of the carotid gland has been investigated recently by 
Kohn, who finds that it is associated with the sympathetic nervous 
system in the same way as the suprarenals. He desires, in fact, to 
make a separate category for such nerve-glands, or paraganglia, as he 
calls them, and considers them all to be derivatives of the sympathetic 
nervous system, and to have nothing to do with excretory organs. The 
carotid gland is, according to him, the foremost of the suprarenal 
masses in the Elasmobranchs, viz. the so-called axillary heart. 
In my opinion, nests of sympathetic ganglion-cells necessarily 
mean the supply of efferent fibres to some organ, for all such ganglia 
are efferent, and also, if they are found in the organ, would have been 
brought into it by way of the blood-vessels supplying the organ, so 
that Aichel’s statement of the origin of the suprarenals in the 
Elasmobranchs seems to me much more probable than a derivation 
from nerve-cells. If, then, it prove that Aichel is right as to the 
origin of the suprarenals, and Kohn is right in classifying the carotid 
gland with the suprarenals, then Maurer’s statements would bring 
the parathyroids, thymus, etc., into line with the adrenals, and sug- 
gest that they represent the segmented glandular excretory organs of 
the branchial region, into which, just as in the interrenals of Elasmo- 
branchs, or the cortical part of the adrenals of the higher vertebrates, 
there has been no invasion of sympathetic ganglion-cells. 
Wheeler makes a most suggestive remark in his paper on Petro- 
myzon: he thinks he has obtained evidence of serial homologues of 
the pronephric tubules in the branchial region of Ammoccetes, but 
has not been able up to the present to follow them out. If what 
he thinks to be serial homologues of the pronephric tubules in 
the branchial region should prove to be the origin of the thymus 
glands of Schaffer, then van Wijhe’s suggestion that the thymus 
represents the excretory organs of the branchial region would 
gain enormously in probability. Until some such further investiga- 
tion has been undertaken, I can only say that it seems to me most 
likely that the thymus, etc., represent the lymphatic branchial glands 
of the Crustacea, and therefore represent the missing coxal glands of 
the branchial] region. 
This, however, is not all, for the appendages of the mesosomatic 
region, as I have shown, do not all bear branchie; the foremost or 
