The Origin of Vertebrates

by Walter Holbrook Gaskell

A review of the animal kingdom as a whole leads to the conclusion that the upward development of animals from an original cœlenterate stock, in which the central nervous system consists of a ring of nervous material surrounding the mouth, has led, in consequence of the elaboration of the central nervous system, to a general plan among the higher groups of invertebrates in the topographical arrangement of the important organs. The mouth is situated ventrally, and leads by means of the œsophagus into an alimentary canal which is situated dorsally to the central nervous system. Thus the œsophagus pierces the central nervous system and divides it into two parts, the supra-œsophageal ganglia and the infra-œsophageal ganglia. This is an almost universal plan among invertebrates, but apparently does not hold for vertebrates, for in them the central nervous system is always situated dorsally and the alimentary canal ventrally, and there is no piercing of the central nervous system by an œsophagus.

Yet a remarkable resemblance exists between the central nervous system of the vertebrate and that of the higher invertebrates, of so striking a character as to compel one school of anatomists to attempt the derivation of vertebrates from annelids. Now, the central nervous system of vertebrates forms a hollow tube, and a diverticulum of this hollow tube, known as the infundibulum, passes to the ventral surface of the brain in the very position where the œsophagus would have been if that brain had belonged to an annelid or an arthropod. This school of anatomists therefore concluded that this infundibular tube represented the original invertebrate œsophagus which had become closed and no longer opened into the alimentary canal owing to the formation of a new mouth in the vertebrate. As, however, the alimentary canal of the vertebrate is ventral to the central nervous system, and not dorsal, as in the invertebrate, it follows that the remains of the original invertebrate mouth into which the œsophagus (in the vertebrate the infundibular tube) must have opened must be searched for on the dorsal side of the vertebrate; and so the theory was put forward that the vertebrate had arisen from the annelid by the reversal of surfaces, the back of the one animal becoming the front of the other.

The difficulties in the way of accepting such reversal of surfaces have proved insuperable, and another school has arisen which suggests that evolution has throughout proceeded on two lines, the one forming groups of animals in which the central nervous system is pierced by the food-channel and the gut therefore lies dorsally to it, the other in which the central nervous system always lies dorsally to the alimentary canal and is not pierced by it. In both cases the highest products of the evolution have become markedly segmented animals, in the former, annelids and arthropods; in the latter, vertebrates. The only evidence on which such theory is based is the existence of low forms of animals, known as the Enteropneusta, the best known example of which is called Balanoglossus; they are looked upon as aberrant annelid forms by many observers.

This theory does not attempt to explain the peculiarities of the tube of the vertebrate central nervous system, or to account for the extraordinary resemblance between the structure and arrangement of the central nervous systems of vertebrates and of the highest invertebrate group.

Neither of these theories is satisfactory or has secured universal acceptance. The problem must be considered entirely anew. What are the guiding principles in this investigation?

The evolution of animal life on this earth can clearly, on the whole, be described as a process of upward progress culminating in the highest form—man; but it must always be remembered that whole groups of animals such as the Tunicata have been able to survive owing to a reverse process of degeneration.

If there is one organ more than another which increases in complexity as evolution proceeds, which is the most essential organ for upward progress, surely it is the central nervous system, especially that portion of it called the brain. This consideration points directly to the origin of vertebrates from the most highly organized invertebrate group—the Arthropoda—for among all the groups of animals living on the earth in the present day they alone possess a central nervous system closely comparable with that of vertebrates. Not only has an upward progress taken place in animals as a whole, but also in the tissues themselves a similar evolution is apparent, and the evidence shows that the vertebrate tissues resemble more closely those of the arthropod than of any other invertebrate group.

The evidence of geology points to the same conclusion, for the evidence of the rocks shows that before the highest mammal—man—appeared, the dominant race was the mammalian quadruped, from whom the highest mammal of all—man—sprung; then comes, in Mesozoic times, the age of reptiles which were dominant when the mammal arose from them. Preceding this era we find in Carboniferous times that the amphibian was dominant, and from them the next higher group—the reptiles—arose. Below the Carboniferous come the Devonian strata with their evidence of the dominance of the fish, from whom the amphibian was directly evolved. The evidence is so clear that each succeeding higher form of vertebrate arose from the highest stage reached at the time, as to compel one to the conclusion that the fishes arose from the race which was dominant at the time when the fishes first appeared. This brings us to the Silurian age, in which the evidence of the rocks points unmistakably to the sea-scorpions, king-crabs, and trilobites as being the dominant race. It was preceded by the great trilobite age, and the whole period, from the first appearance of the trilobite to the time of dwindling away of the sea-scorpions, may be designated the Palæostracan age, using the term Palæostraca to include both trilobites and the higher scorpion and king-crab forms evolved from them. The evidence of geology then points directly and strongly to the origin of vertebrates from the Palæostraca—arthropod forms which were not crustacean and not arachnid, but gave origin both to the modern-day crustaceans and arachnids. The history of the rocks further shows that these ancient fishes, when they first appeared, resembled in a remarkable manner members of the palæostracan group, so that again and again palæontologists have found great difficulty in determining whether a fossil is a fish or an arthropod. Fortunately, there is still alive on the earth one member of this remarkable group—the Limulus, or King-Crab. On the vertebrate side the lowest non-degenerate vertebrate is the lamprey, or Petromyzon, which spends a large portion of its existence in a larval stage, known as the Ammocœtes stage of the lamprey, because it was formerly considered to be a separate species and received the name of Ammocœtes. The larval stages of any animal are most valuable for the study of ancestral problems, so that it is most fortunate for the solution of the ancestry of vertebrates that Limulus on the one side and Ammocœtes on the other are available for thorough investigation and comparison. There are no trilobites still alive, but in Branchipus and Apus we possess the nearest approach to the trilobite organization among living crustaceans.

So strongly do all these different lines of argument point to the origin of vertebrates from arthropods as to make it imperative to reconsider the position of that school of anatomists who derived vertebrates from annelids by reversing the back and front of the animal. Let us not turn the animal over, but re-consider the position, the infundibular tube of the vertebrate still representing the œsophagus of the invertebrate, the cerebral hemispheres and basal ganglia the supra-œsophageal ganglia, the crura cerebri the œsophageal commissures, and the infra-infundibular part of the brain the infra-œsophageal ganglia. It is immediately apparent that just as the invertebrate œsophagus leads into the large cephalic stomach, so the infundibular tube leads into the large cavity of the brain known as the third ventricle, which, together with the other ventricles, forms in the embryo a large anterior dilated part of the neural tube. In the arthropod this cephalic stomach leads into the straight narrow intestine; in the vertebrate the fourth ventricle leads into the straight narrow canal of the spinal cord. In the arthropod the intestine terminates in the anus; in the vertebrate embryo the canal of the spinal cord terminates in the anus by way of the neurenteric canal. Keep the animal unreversed, and immediately the whole mystery of the tubular nature of the central nervous system is revealed, for it is seen that the nervous matter, which corresponds bit by bit with that of the arthropod, has surrounded to a greater or less extent and amalgamated with the tube of the arthropod alimentary canal, and thus formed the so-called central nervous system of the vertebrate.

The manner in which the nervous material has invaded the walls of the tube is clearly shown both by the study of the comparative anatomy of the central nervous system in the vertebrate and also by its development in the embryo.