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 lias 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 branchire ; the foremost or 



