The Origin of Vertebrates

by Walter Holbrook Gaskell

Such an interpretation is, I think, borne out by the study of the most ancient forms of fish. In Bothriolepis, according to Patten, and in Drepanaspis, according to Traquair, the cloacal region and anus follow immediately upon the posterior end of the head-shield, i.e. immediately after that region which presumably contained the branchiæ. Similarly, on the invertebrate side, all those forms which resembled Limulus must have possessed a very short region between the branchial and cloacal parts of the body. The original cloacal part of the vertebrate gut may well have been the original cloaca of the arthropod, into which its intestine emptied itself, especially when we see the tendency of the scorpion group of animals to form an accessory cloacal pouch known as the stercoral pouch or pocket.

Again, it is striking to see how, in certain of the scorpion group, e.g. Thelyphonus and Phrynus, there is a caudal massing of the central nerve-cells as well as a cephalic massing, so that their central nervous system is composed of a cephalic and caudal brain. These two brains are connected together by commissures extending the whole length of the body, in which I have been unable to find any sign of ganglion-cells. What this caudal brain innervates I do not know; it is, I think, a matter worth further investigation, especially as there are many indications in the vertebrate that the lumbo-sacral region of the cord possesses higher functions than the thoracic region.

The method of formation of the alimentary canal as indicated by its innervation is as follows:—

In front an oral chamber, formed, as already pointed out, by the modification of the prosomatic appendages, followed by a respiratory chamber, the muscles and branchiæ of which were the muscles and branchiæ of the mesosomatic appendages. This mesosomatic, or branchial, part was in close contiguity to the cloaca and anus, being separated from it only by a short tube formed in the metasomatic or pronephric region.

I imagine that this connection was originally in the form of an open groove, as already explained for both notochord and the anterior part of the gut itself in Ammocœtes; an open groove formed from the mid-ventral surface of the body, on each side of which were the remnants of the pronephric appendages. By the closure of this groove ventrally, and the growing round of the pleural folds, as already suggested, the remains of the pronephric appendages are indicated by the segmental duct and the form of the vertebrate body is attained.

Even in the branchial region the same kind of thing must, I think, have occurred. The grooved ventral surface became a tube, on each side of which were lying in regular order the in-sunk branchial appendages, the whole being subsequently covered by the pleural folds to form an atrial chamber. A tube thus formed from the grooved ventral surface would carry with it to the new ventral surface the longitudinal venous sinuses, and thus form, in the way already suggested, the heart and ventral aorta. Posterior to the heart in the pronephric region, the same process would give rise to the sub-intestinal vein.

The evidence of comparative anatomy bears out most conclusively the suggestion that in the original vertebrate the gut was mainly a respiratory chamber. In man and all mammals the oral chamber opens into a small pharynx, followed by the œsophagus, stomach and small intestine. Of this whole length, a very small part is taken up by the pharynx, in which, in the embryo, the branchial arches are found, showing that this represents the original respiratory part of the gut. In the ordinary fish this branchial part is much more conspicuous, occupies a large proportion of the gut, and in the lowest fishes, such as Ammocœtes and Amphioxus, the branchial region extends over a large portion of the animal, while the intestine proper is a straight tube, the length of which is insignificant in comparison with its length in the higher vertebrates.

Such a tube was able to act as a digestive tube, owing, as already pointed out, to the digestive powers of the skin-epithelium, and I imagine at first the respiratory chamber, seeing that it composed very nearly the whole of the gut, was at the same time the main digestive chamber; even in Ammocœtes its digestive power is superior to that of the intestine itself.

Just posterior to the branchial part a diverticulum of the gut was formed at an early stage, as seen in Amphioxus, and provided the commencement of the liver. This simple liver-diverticulum became the tubular liver of Ammocœtes, and formed, curiously enough, not a glandular organ of the same character as the liver of the higher vertebrates, but a hepato-pancreas, like the so-called liver of the arthropods, which also is a special diverticulum of the gut, or rather the main true gut of the animal. In both cases the liver is the chief agent in digestion, for in Ammocœtes the liver-extract is very much more powerful in the digestion of proteids than the extract of any other organ tried by Miss Alcock. Subsequently in the vertebrate the gastric and pancreatic glands arise and relieve the liver of the burden of proteid digestion.

It is, to my mind, somewhat significant that the liver on its first formation in the vertebrate should have arisen as a digestive organ of the same character as the so-called liver in the arthropods; whether it originally belonged to any separate segment is in our present state of knowledge difficult to say.