The Origin of Vertebrates

by Walter Holbrook Gaskell

As to the auditory apparatus itself, we see that the elaborate organ for hearing—the cochlea—has been evolved in the vertebrate phylum itself. In the lowest vertebrates the auditory apparatus tends more and more to resolve itself into a simple epithelial sac, the walls of which in places bear auditory hairs projecting into the sac, and in part form otoliths. Such a simple sac forms the early stage of the auditory vesicle in Ammocœtes, according to Shipley; subsequently, by a series of foldings and growings together, the chambers of the ear of the adult Petromyzon, as figured and described by Retzius, are formed. Further, we see that throughout the Vertebrata this sac was originally open to the exterior, the auditory vesicle being first an open pit, which forms a vesicle by the approximating of its sides, the last part to close being known as the recessus labyrinthicus; in many cases, as in elasmobranchs, this part remains open, or communicates with the exterior by means of the ductus endolymphaticus.

Judging, therefore, from the embryological evidence, it would appear that the auditory organ originated as a special sense-organ, formed by modified epithelial cells of the surface, which epithelial surface becoming invaginated, came to line a closed auditory vesicle under the surface. This special sense-organ was innervated from a large ganglionic mass of nerve-cells, situated close against the peripheral sense-cells, the axis-cylinder processes of which formed the sensory roots of the nerve.

Yet another peculiarity of striking significance is seen in connection with the auditory organ of Ammocœtes. The opening of the cartilaginous capsule towards the brain is a large one (Fig. [154]), and admits the passage not only of the auditory and facial nerves, but also of a portion of the peculiar tissue which surrounds the brain. The large cells of this tissue, with their feebly staining nuclei and the pigment between them, make them quite unmistakable; and, as I have already stated, nowhere else in the whole of Ammocœtes is such a tissue found. When I first noticed these cells within the auditory capsule, it seemed to me almost impossible that my interpretation of them as the remnant of the generative and hepatic tissue which surrounds the brain of animals such as Limulus could be true, for it seemed too unlikely that a part of the generative system could ever have become included in the auditory capsule. Still, they are undoubtedly there; and, as already argued with respect to the substance round the brain, they must represent some pre-existing tissue which was functional in the ancestor of Ammocœtes. If my interpretation is right, this tissue must be generative and hepatic tissue, and its presence in the auditory capsule immediately becomes a most important piece of evidence, for it proves that the auditory organ must have been originally so situated that a portion of the generative and hepatic mass surrounding the cephalic region of the nervous system followed the auditory nerve to the peripheral sense-organ.

Fig. 154.—Transverse Section through Auditory Capsules and Brain of Ammocœtes.

Au., auditory organ; VIII, auditory nerve; gl., ganglion cells of VIIIth nerve; Au. cart., cartilaginous auditory capsule; gen., cells of old generative tissue round brain and in auditory capsule; bl., blood-vessels

Here there was a test of the truth of my theory ranking second only to the test of the median eyes; the strongest possible evidence of the truth of any theory is given when by its aid new and unexpected facts are brought to light. The theory said that in the group of animals from which the vertebrates arose, a special sense-organ of the nature of an auditory organ must have existed on the base of one of the appendages situated at the junction of the prosoma and mesosoma, and that into this basal part of the appendage a portion of the cephalic mass of generative and hepatic material must have made its way in close contiguity to the nerve of the special organ.

The only living example which nearly approaches the ancient extinct forms from which, according to the theory, the vertebrates arose, is Limulus, and, as has already been shown, in this animal, in the very position postulated by the theory, a large special sense-organ—the flabellum—exists, which, as already stated, may well have given rise to a sense-organ concerned with equilibration and audition. If, further, it be found that a diverticulum of the generative and hepatic material does accompany the nerve of the flabellum in the basal part of the appendage, then the evidence becomes very strong that the auditory organ of Ammocœtes, i.e. of the ancient Cephalaspids, was derived from an organ homologous with the flabellum; that, therefore, the material round the brain of Ammocœtes was originally generative and hepatic material; that, in fact, the whole theory is true, for all the parts of it hang together so closely that, if one portion is accepted, all the rest must follow. As pointed out in my address at Liverpool, and at the meeting of the Philosophical Society at Cambridge, it is a most striking fact that a mass of the generative and hepatic tissue does accompany the flabellar nerve into the basal part of this appendage. Into no other appendage of Limulus is there the slightest sign of any intrusion of the generative and hepatic masses; nowhere, except in the auditory capsule, is there any sign of the peculiar large-celled tissue which surrounds the brain and upper part of the spinal cord of Ammocœtes. The actual position of the flabellum on the basal part of the ectognath is shown in Fig. [155], A, and in Fig. [155], B, I have removed the chitin, to show the generative and hepatic tissue (gen.) lying beneath.

The reason why, to all appearance, the generative and hepatic mass penetrates into the basal part of this appendage only is apparent when we see (as Patten and Redenbaugh have pointed out) to what part of the appendage the flabellum in reality belongs.