The Origin of Vertebrates

by Walter Holbrook Gaskell

Fig. 46.—The Brain, Eyes, and Anterior Termination of the Alimentary Canal of Artemia, viewed from the Dorsal Aspect.

Br., brain; l.e., lateral eyes; c.e., median eyes; Al., alimentary canal.

Fig. 47.—A, The Formation of the Retina of the Eye of Ammocœtes (after Scott); B, The Formation of the Retina of the Eye of Ammocœtes, on my theory.

R., retina; l., lens; O.n., optic nerve fibres; Al., cephalic end of invertebrate alimentary canal; V., cavity of ventricles of brain; Al.d., anterior diverticulum of alimentary canal; op.d., optic diverticulum.

My friend Mr. W. B. Hardy has especially investigated the nervous system of Artemia. In the course of his work he cut serial sections through the whole animal, and, as mentioned in my paper in the Journal of Anatomy and Physiology, he discovered that the retinal ganglion of each lateral eye is so closely attached to the end of the corresponding diverticulum of the gut that the lining cells of the ventral part of the diverticulum form a lining to the retinal ganglion (Fig. [45]). In this animal there are only two gut-diverticula, which are situated most anteriorly. I have plotted out this series of sections by means of a camera lucida, with the result that the retina appears as a bulging attached ventro-laterally to the extremity of each gut-diverticulum, as is shown in Fig. [46]. It is instructive to compare with this figure Scott's picture of the developing eye in Ammocœtes, where he figures the retina as a bulging attached ventrally to the extremity of the narrow tube of the optic diverticulum. In Fig. [47], A, I reproduce this figure of Scott, and by the side of it, Fig. [47], B, I have represented the origin of the vertebrate eye as I believe it to have occurred.

We see, then, this very striking fact, that in the most primitive of the Crustacea, not only are there two anterior diverticula of the gut, but also the retinal ganglion of the lateral eye is in specially close connection with the end of the diverticulum on each side. In fact, we find in the nearest living representative of the trilobites a retina and retinal ganglion and optic nerve, closely resembling that of the vertebrate, in close connection with an epithelial tube which has nothing to do with the organ of sight, but is one of a pair of anterior gut-diverticula. It is impossible to obtain more decisive evidence that the trilobites possessed a pair of gut-diverticula surrounded to a greater or less extent by the retina and optic nerve of each lateral eye.

Such anterior diverticula are commonly found in the lower Crustacea; they are usually known by the name of liver-diverticula, but as they take no part in digestion, and, on the contrary, represent that part of the gut which is most active in absorption, the term liver is not appropriate, and it is therefore better to call them simply the pair of anterior diverticula. Our knowledge of their function in Daphnia is given in a paper by Hardy and M‘Dougall, which does not appear to be widely known. Hardy succeeded in feeding Daphnia with yolk of egg in which carmine grains were mixed, and was able in the living animal to watch the whole process of deglutition, digestion, and absorption. The food, which is made into a bolus, is moved down to the middle region of the gut, and there digestion takes place. Then by an antiperistaltic movement the more fluid products of the digestion-process are sent right forward into the two anterior diverticula, where the single layer of columnar cells lining these diverticula absorbs these products, the cells becoming thickly studded with fat-drops after a feed of yolk of egg. The carmine particles, which were driven forward with the proteid- and fat-particles, are not absorbed, but are at intervals driven back by contractions of the anterior diverticula to the middle region of the gut.

These observations prove most clearly that the anterior diverticula have a special nutrient function, being the main channels by which new nutrient material is brought into the body, and, as pointed out by the authors, it is a remarkable exception in the animal kingdom that absorption should occur in that portion of the gut which is anterior to the part in which digestion occurs. In all these animals the two anterior diverticula extend forwards over the brain, and, as we have seen in Artemia, the anterior extremity of each one is so intimately related to a part of the brain—viz. the retinal ganglion—as to form a lining membrane to that mass of nerve-cells. It follows, therefore, that the nutrient fluid absorbed by the cells of this part of the gut-diverticulum must be primarily for the service of the retinal ganglion. In fact, the relations of this anterior portion of the gut to the brain as a whole suggest strongly that the marked absorptive function of this anterior portion of the gut exists in order to supply nutrient material in the first place to the most vital, most important organ in the animal—the brain and its sense-organs. This conclusion is borne out by the fact that in these lower crustaceans the circulation of blood is of a very inefficient character, so that the tissues are mainly dependent for their nutrition on the fluid immediately surrounding them. It stands to reason that the establishment of the anterior portion of the gut as a nutrient tube to the brain would necessitate a closer and closer application of the brain to that tube, so that the process of amalgamation of the brain with the single layer of columnar epithelial cells which constitutes the wall of the gut (which we see in its initial stage in the retinal ganglion of Artemia), would tend rapidly to increase as more and more demands were made upon the brain, until at last both the supra- and infra-œsophageal ganglia, as well as the retinal ganglia and optic nerves, were in such close intimate connection with the ventral wall of the anterior portion of the gut and its diverticula as to form a brain and retina closely resembling that of Ammocœtes.

Such an origin for the lateral eyes of the vertebrate explains in a simple and satisfactory manner why the vertebrate retina is a compound retina, and why both retina and optic nerve have an apparent tubular development.