950 REPORT— 1896. 
also with the large supra-cesophageal mass. In the middle of this large brain 
mass a small canal is seen closely surrounded and compressed with nervous 
matter, as is shown in this specimen of Thelyphonus; this canal is the alimentary 
canal. Again, Hardy, in his work on the nervous system of Crustacea, has sections 
through the brain of Branchipus which demonstrate so close an attachment between 
the nervous matter of the optic ganglion and the anterior diverticulum of the gut 
that uo line of demarcation is visible between the cells of the gut wall and the 
cells of the optic ganglion. 
For all these reasons I consider that the tubular nature of the vertebrate 
central nervous system is explained by my hypothesis much more satisfactorily 
and fully than by any other as yet put forward; it further follows that if this 
hypothesis enables us to homologise all the other parts of the head region of the 
vertebrate with similar parts in the arthropod, then it ceases to be an hypothesis, 
but rises to the dignity of the most probable theory of the origin of vertebrates. 
Origin of Segmental Cranial Nerves. 
1. The phylogenetic test.—It follows from the close resemblance of the brain 
region of the central nervous systems in the two groups of animals that the cranial 
nerves of the vertebrate must be homologous with the foremost nerves of such an 
animal as Limulus, and must therefore supply bomologous organs. Leaving out 
of consideration for the present the nerves of special sense, it follows that the seg- 
mental cranial nerves must be divisible into two groups corresponding to two sets 
of segmental muscles, viz. a group supplying structures homologous to the appen- 
dages of Limulus and its allies, and a group supplying the somatic or body muscles ; 
in other words, we must find precisely what is the most marked characteristic of 
the vertebrate cranial nerves, viz. that they are divisible into two sets correspond- 
ing to a double segmentation in the head region. The one set, consisting of the 
Vth, VIIth, [Xth, and Xth nerves, supply the muscles of the branchial or 
visceral segments; the other set, consisting of the IlIrd, IVth, Vith, and XIfth 
nerves, the muscles of the somatic segments. Further, we see that the nerves 
supplying the branchial segments, like the nerves supplying the appendages in 
Limulus, are mixed motor and sensory, while the nerves supplying the somatic 
segments are all purely motor, the corresponding sensory nerves running separately 
as the ascending root of the fifth nerve; so also in Limulus, the nerves supplying 
the powerful body muscles arise separately from those supplying the appendages, 
and also are quite separate from the purely sensory or epimeral (Milne Edwards) ’ 
nerves which supply the surfaces of the carapace in the prosomatic and mesoso- 
matic regions. Finally, the researches of Hardy* have shown that the motor portion 
of these appendage nerves, just like the nerves of the branchial segmentation in 
vertebrates, 7.e. the motor part of the trigeminal, of the facial, of the glosso-pharyngeal, 
and of the vagus, arise from nerve centres or nuclei quite separate from those 
which give origin to the motor nerves of the somatic muscles. The phylogenetic 
history, then, of the cranial nerves points directly to the conclusion that the Vth, 
Vilth, [Xth, and Xth nerves originally innervated structures of the nature of 
arthropod appendages. 
We can, however, go further than this, for we find, as we trace downwards 
throughout the vertebrate kingdom the structures supplied by these nerves, that 
they are divisible into two well-marked groups, especially well seen in Ammo- 
coetes, viz. :-— 
1. A posterior group, viz. the VIIth, IXth, and Xth nerves, which arise 
from oe medulla oblongata and supply all the structures within a branchial 
chamber. 
2. An anterior group, viz. the Vth nerves, which arise from the hind brain 
and supply all the structures within an oral chamber. 
1 Milne Edwards, ‘Recherches sur l’Anatomie des Limulus,’ Ann. des Sc. Nat., 
5th ser. 
2 Hardy, Phil. Trans. Roy. Soc. 1894. 
