THE REGION OF THE SPINAL CORD 429 



opercular appendage carries the thyroid gland. Again, the basal part 

 of the appendage is all that is left ; the thyroid gland is in position a 

 coxal gland. It ought, therefore, to represent the coxal gland of this 

 appendage, just as the thymus, tonsils, etc., represent the coxal glands 

 of the rest of the mesosoniatic appendages. In the thyroid gland we 

 again see a ductless gland of immense importance to the economy, 

 not a useless organ, but one, like the other modified coxal glands, 

 whose removal involves far-reaching vital consequences. Such a 

 gland, on my theory, was in the arthropod a part of the external genital 

 ducts which opened on the basal joint of the operculum. What, then, 

 is the opinion of morphologists as to the meaning of these external 

 genital ducts ? 



In a note to Gulland's paper on the coxal glands of Limulus, 

 Lankester states that the conversion of an externally-opening tubular 

 gland (coxal gland) into a ductless gland is the same kind of thing 

 as the history of the development of the suprarenal from a modified 

 portion of mesonephros, as given by Weldon. Further, that in other 

 arthropods with glands of a tubular character opening to the exterior 

 at the base of the appendages, we also have coxal nephridia, such 

 as the shell-glands of the Entomostraca, green glands of Crustacea 

 (antennary coxal gland) ; and further on he writes : " When once the 

 notion is admitted that ducts opening at the base of limbs in the 

 Arthropoda are possibly and even probably modified nephridia, we 

 immediately conceive the hypothesis that the genital ducts of the 

 Arthropoda are modified nephridia." 



So, also, Korschelt and Heider, in their general summing up on 

 the Arthropoda, say : " In Peripatus, where the nephridia appear, as 

 in the Annelida, in all the trunk-segments, a considerable portion of 

 the primitive segments is directly utilized for the formation of the 

 nephridia. In the other groups, the whole question of the rise of 

 the organs known as nephridia is still undecided, but it may be 

 mentioned as very probable that the salivary and anal glands of 

 I'eripatus, the antennal and shell-glands of the Crustacea, the coxal 

 glands of Limulus and the Arachnida, as well as the efferent genital 

 ducts, are derived from nephridia, and in any case are mesodermal 

 in origin." 



The necessary corollary to this exceedingly probable argument is 

 that glandular structures such as the uterine glands of the scorpion 

 already described, which are found in connection with these terminal 



