The Origin of Vertebrates

by Walter Holbrook Gaskell

This long-continued free-living existence in the larval or Ammocœtes stage makes the lamprey, more than any other type of lowly organized fish, invaluable for the present investigation, for throughout the animal kingdom it is recognized that the larval form approaches nearer to the ancestral type than the adult form, whether the latter is progressive or degenerate. Not only are the tissues formed during the stages which are passed through in a free-living larval form, serviceable tissues comparable to those of adult life, but also these stages proceed at so much slower a rate than do those in the embryo in utero or in the egg, as to make the larval form much more suitable than the embryo for the investigation of ancestral problems. It is true enough that the free life of the larva may bring about special adaptations which are not of an ancestral character, as may also occur during the life of the adult; but the evidence is very strong that although some of the peculiarities of the larva may be due to such cœnogenetic factors, yet on the whole many of them are due to ancestral characters, which disappear when transformation takes place, and are not found in the adult.

Thus if it be supposed that the amphibian arose from the fish, the tadpole presents more resemblance to the fish than the frog. If it be supposed that the arthropod arose from the segmented worm, the caterpillar bears out the suggestion better than the adult imago. If it be supposed that the tunicate arose from a stock allied to the vertebrate, it is because of the peculiarities of the larva that such a supposition is entertained. So, too, if it be supposed that the fish arose from a member of the arthropod group, the larval form of the fish is most likely to give decisive information on the point.

For all these reasons the lowest form of fish to be investigated, in the hopes of finding out the nature of the earliest formed fish, is not Amphioxus, but Ammocœtes, the larval form of the lamprey—a form which, as I hope to satisfy my reader after perusal of subsequent pages, more nearly resembles the ancient Cephalaspidian fishes than any other living vertebrate.

Comparison of Central Nervous Systems of Vertebrate and Arthropod without Reversal of Surfaces.

So far different lines of investigation all point to the origin of the vertebrate from arthropods, the group of arthropods in question being now extinct, the nearest living representative being Limulus; also to the fact that of the two theories of the origin of vertebrates, that one which is based on the resemblance between the central nervous systems of the Vertebrata and the Appendiculata (Arthropoda and Annelida) is more in accordance with this evidence than the other, which is based mainly on the supposed possession of a notochord among certain animals.

How is it, then, that this theory has been discredited and lost ground? Simply, I imagine, because it was thought to necessitate the turning over of the animal. Let us, then, again look at the nervous system of the vertebrate, and see whether there is any such necessity.

As previously mentioned, the comparison of the two central nervous systems showed such close resemblances as to force those anatomists who supported this theory to the conclusion that the infundibular tube was in the position of the original œsophagus; they therefore looked for the remains of a mouth opening in the dorsal roof of the brain, but did not attempt to explain the extraordinary fact that the infundibular tube is only a ventral offshoot from the tube of the central nervous system. Yet this latter tube is one, if not the most striking, of the peculiarities which distinguish the vertebrate; a tubular central nervous system such as that of the vertebrate is totally unlike any other nervous system, and the very fact that the two nervous systems of the vertebrate and arthropod are so similar in their nervous arrangements, makes it still more extraordinary that the nervous system should be grouped round a tube in the one case and not in the other.

Now, in the arthropod the œsophagus leads directly into the stomach, which is situated in the head-region, and from this a straight intestine passes directly along the length of the body to the anus, where it terminates. The relations of mouth, œsophagus, alimentary canal, and nervous system in these animals are represented in the diagram (Fig. [3]).

Any tube, therefore, such as that of the infundibulum, which would represent the œsophagus of such an animal, must have opened into the mouth on the ventral side, and into the stomach on the dorsal side, and the lining epithelium of such an œsophagus must have been continuous with that of the stomach, and so of the whole intestinal tract.

Supposing, then, the animal is not turned over, but that the dorsal side still remains dorsal and ventral ventral, then the original mouth-opening of the œsophagus must be looked for on the ventral surface of the vertebrate brain in the region of the pituitary body or hypophysis, and on the dorsal side the tube representing the œsophagus must be continuous with a large cephalically dilated tube, which ought to pass into a small canal, to run along the length of the body and terminate in the anus.