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 mesosomatic 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 
Peripatus, 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 
