216 THE ORIGIN OF VERTEBRATES 
SUMMARY. 
Step by step in the preceding chapters the evidence is accumulating in 
favour of the origin of vertebrates from a member of the paleostracan group. 
In a continuously complete and harmonious manner the evidence has throughout 
been most convincing when the vertebrate chosen for the purpose of my argu- 
ments has been Ammoceetes. 
So many fixed points have been firmly established as to enable us to proceed 
further with very great confidence, in the full expectation of being able 
ultimately to homologize the Vertebrata with the Paleostraca even to minute 
details. 
Perhaps the most striking and unexpected result of such a comparison is the 
discovery that the thyroid gland is derived from the uterus of the paleostracan 
ancestor. Yet so clear is the evidence that it is difficult to see how the homology 
can be denied. 
In the one animal (Paleostraca) the foremost pair of mesosomatic appendages 
forms the operculum, which always bears the terminal generative organs and 
is fused in the middle line. In many forms, essentially in Eurypterus and the 
ancient sea-scorpions, the operculum was composed of two segments fused 
together: an anterior one which carried the uterus, and a posterior one which 
carried the first pair of branchie. 
In the other animal (Ammoceetes) the foremost segments of the mesosomatic 
or respiratory region, immediately in front of the glossopharyngeal segments, 
are supplied by the facial nerve, and are markedly different from those supplied 
by the vagus and glossopharyngeal, for the facial supplies two segments fused 
together; the anterior one, the thyroid segment, carrying the thyroid gland, 
the posterior one, the hyoid segment, carrying the first pair of branchie. 
Just as in Hurypterus the fused segment, carrying the uterus on its internal 
surface, forms a long median tongue which separates the most anterior branchial 
segments on each side, so also the fused segment carrying the thyroid forms in 
Ammocetes a long median tongue, which separates the most anterior branchial 
segments on each side. 
Finally, and this is the most conclusive evidence of all, this thyroid gland 
of Ammoccetes is totally unlike that of any of the higher vertebrates, and, 
indeed, of the adult form Petromyzon itself, but it forms an elaborate com- 
plicated organ, which is directly comparable with the uterus and genital ducts 
of animals such as scorpions. Not only is such a comparison valid with respect to 
its shape, but also with respect to its structure, which is absolutely unique among 
vertebrates, and very different to that of any other vertebrate gland, but 
resembles in a striking manner a glandular structure found in the uterus, both 
of male and female scorpions. 
The generative glands in Limulus, together with the liver-glands, form a 
large glandular mass, situated in the head-region closely surrounding the central 
nervous system, so that the genital ducts pass from the head-region tailwards 
to the operculum. In the scorpion they lie in the abdominal region, so that 
their ducts pass headwards to the operculum. 
Probably in the Paleostraca the generative mass was situated in the cephalic 
region as in Limulus, and it is probable that the remnant of it still exists in 
