Probably in the Palæostraca the generative mass was situated in the cephalic region as in Limulus, and it is probable that the remnant of it still exists in { 217}Ammocoetes in the shape of the peculiar large cells packed together, with pigment masses in between them, which form such a characteristic feature of the glandular-looking material, which fills up the space between the cranial walls and the central nervous system.
Finally, the relationship which has been known from time immemorial to exist between the sexual organs and the thyroid in man and other animals, and has hitherto been a mystery without any explanation, may possibly be the last reminiscence of a time when the thyroid glands were the uterine glands of the palæostracan ancestor.
The consideration of the facial nerve, and the segments it supplies, still further points to the origin of the Vertebrata from the Palæostraca.
_THE EVIDENCE OF THE OLFACTORY APPARATUS_
Fishes divided into Amphirhinæ and Monorhinæ.--Nasal tube of the lamprey.--Its termination at the infundibulum.--The olfactory organs of the scorpion group.--The camerostome.--Its formation as a tube.--Its derivation from a pair of antennæ.--Its termination at the true mouth.--Comparison with the olfactory tube of Ammocoetes.--Origin of the nasal tube of Ammocoetes from the tube of the hypophysis.--Direct comparison of the hypophysial tube with the olfactory tube of the scorpion group--Summary.
In the last chapter I finished the evidence given by the consideration of the mesosomatic or opisthotic nerves, and the segments they supplied. The evidence is strongly in accordance with that of previous chapters, and not only confirms the conclusion that vertebrates arose from some member of the Palæostraca, but helps still further to delimit the nature of that member. It is almost startling to see how the hypothesis put forward in the second chapter, suggested by the consideration of the nature of the vertebrate central nervous system and of the geological record, has received stronger and stronger confirmation from the consideration of the vertebrate optic apparatus, the vertebrate skeleton, the respiratory apparatus, and, finally, the thyroid gland. All fit naturally into a harmonious whole, and give a feeling of confidence that a similar harmony will be found upon consideration of the rest of the vertebrate organs.