2l6 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 palseostracan 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 Ammoccetes. 



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 Palfleostraca 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 palfeostracan 

 ancestor. Yet so clear is the evidence that it is difficult to see how the homology 

 can be denied. 



In the one animal (Palasostraca) 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 branchiae. 



In the other animal (Ammoccetes) 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 branchiae. 



Just as in Eurypterus 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 

 Ammoccetes 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 Ammocoetes 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 Palaeostraca the generative mass was situated in the cephalic 

 region as in Limulus, and it is probable that the remnant of it still exists in 



