The Origin of Vertebrates

by Walter Holbrook Gaskell

The observer Spengel, who has made the most exhaustive study of these strange forms, rejects in toto any connection with vertebrates, and considers them rather as aberrant annelids. The so-called evidence of the tubular central nervous system is worth nothing. There is not the slightest sign of any tubular nervous system in the least resembling that of the vertebrate. It is simply that in one place of the collar-region the piece of skin containing the dorsal nerve of the animal, owing to the formation of the collar, is folded, and thus forms just at this region a short tube. My theory explains in a natural manner every portion of the elaborate and complicated tube of the vertebrate central nervous system. In the Balanoglossus theory the evolution of the vertebrate tube in all its details from this collar-fold is simple guesswork, without any reasonable standpoint. Similarly, the small closed diverticulum of the gut in Balanoglossus, which is dignified with the name of "notochord," has no right to the name. As I have already said, it may help to understand why the notochord has such a peculiar structure, but it gives no help to understanding the peculiar position of the notochord. The only really striking resemblance is between the gill-slits of Amphioxus and of the Enteropneusta. In this comparison there is a very great difficulty, very similar to that of the original attempts to derive vertebrates from annelids—the gill-slits open ventrally in the one animal and dorsally in the other. In both animals an atrial cavity exists which is formed by pleural folds, and in these pleural folds the gonads are situated so that the similarity of the two branchial chambers seems at first sight very complete. In the Enteropneusta, however, there are certain forms—Ptychodera—in which these pleural folds have not met in the mid-line in this branchial region, and in these it is plainly visible that these folds, with their gonads, spring from the ventral mid-line and arch over the dorsal region of the body. Equally clearly Amphioxus shows that its pleural folds, with the gonads, spring from the dorsal side of the animal, and grow ventralwards until they fuse in the ventral mid-line (cf. Fig. [168]).

As far, then, as this one single striking similarity between Amphioxus and the Enteropneusta is concerned it necessitates the reversal of dorsal and ventral surfaces to bring the two branchial chambers into harmony.

Fig. 168.—Diagram illustrating the Position of the Pleural Folds and Gonads in Ptychodera (A) and Amphioxus (B) respectively.

Al., alimentary canal; D.A., dorsal vessel; V.A., ventral vessel; g., gonads; NC., notochord; C.N.S., central nervous system.

In a mud-dwelling animal, like Balanoglossus, which possesses no appendages, no special sense-organs, it seems likely enough that ventral and dorsal may be terms of no particular meaning, and consequently what is called ventral in Balanoglossus may correspond to what is dorsal in Amphioxus; in this way the branchial regions of the two animals may be closely compared. Such comparison, however, immediately upsets the whole argument of the vertebrate nature of Balanoglossus based on the relative position of the central nervous system and gut, for now that part of its nervous system which is looked upon as the central nervous system in Balanoglossus is ventral to the gut, just as in a worm-like animal, and not dorsal to it as in a vertebrate.

There is absolutely no possibility whatever of making such a detailed comparison between Balanoglossus and any vertebrate, as I have done between a particular kind of arthropod and Ammocœtes. In the latter case not only the topographical anatomy of the organs in the two animals is the same, but the comparison is valid even to microscopical structure. In the former case the origin of almost all the vertebrate organs is absolutely hypothetical, no clue is given in Balanoglossus, not even to the segmented nature of the vertebrate. The same holds good with the evidence from Embryology and from Palæontology. I have pointed out how strongly the evidence in both cases confirms that of Comparative Anatomy. In neither case is the strength of the evidence for Balanoglossus in the slightest degree comparable. In Embryology an attempt has been made to compare the origin of the cœlom in Amphioxus and in Balanoglossus. In Palæontology there is nothing, only an assumption that in the Cambrian and Lower Silurian times a whole series of animals were evolved between Balanoglossus and the earliest armoured fishes, which have left no trace, although they were able to hold their own against the dominant Palæostracan race. The strangeness of this conception is that, when they do appear, they are fully armoured, as in Pteraspis and Cephalaspis, and it is extremely hard luck for the believers in the Balanoglossus theory that no intermediate less armoured forms have been found, especially in consideration of the fact that the theory of the origin from the Palæostracan does not require such intermediate forms, but finds that those already discovered exactly fulfil its requirements.

One difficulty in the way of accepting the theory which I have advocated is perhaps the existence of the Tunicata. I cannot see that they show any affinities to the Arthropoda, and yet they are looked upon as allied to the Vertebrata. I can only conclude that both they and Amphioxus arose late, after the vertebrate stock had become well established, so that in their degenerated condition they give indications of their vertebrate ancestry and not of their more remote arthropod ancestry.

In conclusion, the way in which vertebrates arose on the earth as suggested in this book carries with it many important far-reaching conclusions with respect to the whole problem of Evolution.

When the study of Embryology began, great hopes were entertained that by its means it would be possible to discover the pedigree of every group of animals, and for this end all the stages of development in all groups of animals were sought for and, as far as possible, studied. It was soon found, however, that the interpretation of what was seen was so difficult, as to give rise to all manner of views, depending upon the idiosyncrasy of the observer. At his will he decided whether any appearance was cœnogenetic or palingenetic, with the result that, in the minds of many, embryology has failed to afford the desired clue.