present time no indication of gill-slits has been found, and we cannot say with certainty how these animals breathed. Further, in the Upper Silurian of Lesmahago, Lanarkshire, a scorpion (_Palæophonus Hunteri_), closely resembling the modern scorpion, has been found, which, as Lankester states, was in all probability aquatic, and not terrestrial in its habits. How it {151}breathed is unknown; it shows no signs of stigmata, such as exist in the scorpion of to-day. Although we possess as yet no certain knowledge of the position of the gill-openings in these ancient scorpion-like forms, what we can say with certainty--and that is the important fact--is, that at the time when the vertebrates appeared, a very large number of the dominant arthropod race possessed internally-situated branchiæ, which had been directly derived from the branchiæ-bearing appendages of their Limulus-like kinsfolk. This abolition of the branchiæ-bearing appendages as external organs of locomotion, with the retention of the important branchial portion of the appendage as internal branchiæ, is a very important suggestion in any discussion of the way vertebrates have arisen from arthropods; for, if the same principle is of universal application, it leads directly to the conclusion that whenever an appendage possesses an organ of vital importance to the animal, that organ will remain, even though the appendage as such completely vanishes. Thus, as will be shown later, special sense-organs such as the olfactory remain, though the animal no longer possesses antennæ; the important excretory organs, the coxal glands, and important respiratory organs, the branchiæ, are still present in the vertebrate, although the appendages to which they originally belonged have dwindled away, or, at all events, are no longer recognizable as arthropod appendages. INNERVATION OF BRANCHIAL SEGMENTS. Passing from _a priori_ considerations to actual facts, it is advisable to commence with the innervation of the branchial segments; for, seeing that the foundation of the whole of this comparative study of the vertebrate and the arthropod is based upon the similarity of the two central nervous systems, it follows that we must look in the first instance to the innervation of any organ or group of organs in order to find out their relationship in the two groups of animals. The great characteristic of the vertebrate branchial organs is their segmental arrangement and their innervation by the vagus group of nerves, _i.e._ by the hindermost group of the cranial segmental nerves. These cranial nerves are divided by Gegenbaur into two great groups--an anterior group, the trigeminal, which supplies the muscles of mastication, and a posterior group, the vagus, which is essentially {152}respiratory in function. Of these two groups, I will consider the latter group first.