The Origin of Vertebrates

by Walter Holbrook Gaskell

The shifting of the nasal tube from a ventral to a dorsal position, as seen in Ammocœtes, is, perhaps, the most important of all clues in connection with the comparison of Ammocœtes to the Palæostracan on the one hand, and to the Cephalaspid on the other; for, whereas the exact counterpart of the opening of such a tube is always found on the dorsal head-shield in all members of the latter group, nothing of the kind is ever found on the dorsal carapace of the former group.

The reason for this difference is made immediately evident in the development of Ammocœtes itself, for the olfactory tube originates as a ventral tube—the tube of the hypophysis—in exactly the same position as the olfactory tube of the Palæostracan, and later on in its development takes up a dorsal position.

In fact, Ammocœtes in its development indicates how the Palæostracan head-shield became transformed into that of the Cephalaspid.

In another most important character Ammocœtes indicates its relationship to the Cephalaspidæ, for it possesses an external skeleton or head-shield composed of muco-cartilage, which is the exact counterpart of the so-called bony head-shield of the latter group; and still more strikingly the structure of the cephalaspidian head-shield is remarkably like that of muco-cartilage. In the one case, by the deposition of calcium salts, a hard external skeleton, capable of being preserved as a fossil, has been formed; in the other, by the absence of the calcium salts, a soft chondro-mucoid matrix, in which the characteristic cells and fibrils are embedded, distinguishes the tissue.

The recognition that the head-shields of these most primitive fishes were not composed of bone, but of muco-cartilage, the precursor of both cartilage and bone, immediately clears up in the most satisfactory manner the whole question of their derivation from elasmobranch fishes; for the main argument in favour of the latter derivation is the exceedingly strong one that bone succeeds cartilage—not vice versâ—therefore, these forms, since their head-shield is bony, must have arisen from some other fishes with a cartilaginous skeleton, most probably of an elasmobranch nature. Seeing, however, that the structure of their shields resembles muco-cartilage much more closely than bone, and that Ammocœtes forms a head-shield of muco-cartilage closely resembling theirs, there is no longer any necessity to derive the jawless fishes from the gnathostomatous; but, on the contrary, we may look with certainty upon the Agnatha as the most primitive group from which the others have been derived.

The history of the rocks shows that the group of fishes, Pteraspis and Cyathaspis, are older than the Cephalaspidæ—come, therefore, phylogenetically between the Palæostraca and the latter group. In this group the head-shields are of a very different character, without any sign of any structure comparable with that of bone, and although they possessed both lateral and median eyes, there is never in any case any trace of a dorsal nasal orifice. Their olfactory passage, like that of the Palæostraca, must have been ventral.

The remarkable comparison which exists between the head-shields of Ammocœtes and Cephalaspis, enables us to locate the position of the brain and cranium of the latter with considerable accuracy, and so to compare the segmental markings found in many of these fossils with the corresponding markings, found either in fossil Palæostraca or on the head-carapaces of living scorpions and spiders, such as Phrynus and Mygale. In all cases the cranial region was covered with a median plate, often especially hard, which corresponded to the glabellum of the trilobite; the growth of the cranium can be traced from its beginnings as the upturned lateral flanges of the plastron to the membranous cranium of Ammocœtes.

From such a comparison it follows that the segments, found in the antero-lateral region of the head-shield, were not segments of the cranium, but of parts beyond the region of the cranium, and from their position must have been segments supplied by the trigeminal nerve, and not by the vagus group; segments, therefore, which did not indicate gills and gill-slits, but muscles, innervated by the trigeminal nerve; muscles which, as indicated by the corresponding markings on the carapace of Phrynus, Mygale, etc., were the tergo-coxal muscles of the prosomatic appendages.

The discovery of the nature of these appendages in the Pteraspidæ and Cephalaspidæ, as well as in the Asterolepidæ (Pterichthys and Bothriolepis), is a problem of the future, though in the latter, not only have the well-known oar-like appendages been long since discovered, but Patten has recently found specimens of Bothriolepis which throw light on the anterior masticating gnathite-like appendages which these ancient forms possessed.

CHAPTER XI