The Origin of Vertebrates

by Walter Holbrook Gaskell

Gaubert does not appear to have seen the goblets at all clearly; he describes them simply as conical eminences, and states that they "recouvrent un pore analogue a celui des poils mais plus petit; il est rempli par le protoplasma de la couche hypodermique." From the ganglion, according to him, nervous prolongations pass, which traverse the chitinogenous layer and terminate at the base of the conical eminences. Each of these prolongations "présente sur son trajet, mais un peu plus près du ganglion que de sa terminaison périphérique, une cellule nerveuse fusiforme (g.) offrant, comme celles du ganglion, un gros noyau." He illustrates his description with the following, Fig. [153], taken from his paper.

Fig. 153 (from Gaubert).—Section of a Tooth of Pecten of Scorpion.

n., nerve; gl., ganglion.

I have not been able to obtain any evidence of a fusiform nerve-cell on the course of the terminal nerve-fibres as depicted by him; fusiform cells there are in plenty, as depicted in my drawing, but none with a large nucleus resembling those of the main ganglion. In no case, either in the flabellum or in the branchial organs of Limulus, or in the pecten-organs, have I ever seen a ganglion-cell within the chitin-layer; all the nuclei seen there resemble those of the cells of the hypodermis or else the elongated nuclei characteristic of the presence of nerve-fibres. Gaubert's drawing is a striking one, and I have looked through my specimens to see whether there was anything similar, but have hitherto failed to obtain any definite evidence of anything of the kind.

I feel, myself, that an exhaustive examination of the structure and function of the pecten of scorpions ought to be undertaken. At present I can only draw the attention of my readers to the similarity of the arrangement of parts, and of the nature of the end-organs, in the sense-organs of the flabellum of Limulus and of the pecten of the scorpion. In both cases the special nerve-fibres terminate in a massive ganglion, situated just below the chitinogenous layer. In both cases the terminal fibres from these ganglion-cells pass through the modified chitinous layer to supply end-organs of a striking character; and although the end-organ of the pecten of the scorpion does not closely resemble the majority of the end-organs of the flabellum, yet it does resemble, on the one hand, the isolated poriferous spikes found on the flabellum (Fig. [149]) and, on the other, the poriferous goblets found on the sense-patches of the branchial appendages of Limulus (Fig. [144], A), so that a combination of these two end-organs would give an appearance very closely resembling that of the pecten of the scorpion.

Finally, the special so-called 'racquet-organs' of Galeodes, which are found on the most basal segments of the last pair of prosomatic appendages, ought also to be considered here. Gaubert has described their structure, and shown how the nerve-trunk in the handle of the racquet splits up into a great number of separate bundles, which spread out fan-shaped to the free edge of the racquet; each of these separate bundles supplies a special sense-organ, which terminates as a conical eminence on the floor of a deep groove, running round the whole free edge of the racquet. This groove is almost converted into a canal, owing to the projection of its two sides. Gaubert imagines that the sense-organs are pushed forward out of the groove to the exterior by the turgescence of the whole organ; each of the nerve-fibres forming a bundle is, according to Gaubert, connected with a nerve-cell before it reaches its termination.

This sketch of the special sense-organs on the appendages of Limulus, of the scorpions, of Galeodes, and other arachnids, and their comparison with the porous chordotonal organs of insects, affords reason for the belief that we are dealing here with a common group of organs, which, although their nature is not definitely known, have largely been accredited with the functions of equilibration and audition, a group of organs among which the origin of the auditory organ of vertebrates must be sought for, upon any theory of the origin of vertebrates from arthropods.

Whenever in any animal these organs are concentrated together to form a special organ, it is invariably found that the nerve going to this organ is very large, out of all proportion to the size of the organ, and also that the nerve possesses, close to its termination in the organ, large masses of nerve-cells. Thus, although the whole hind wing in the blow-fly has been reduced to the insignificant balancers or 'halteres,' yet, as Lowne states, the nerves to them are the largest in the body.

The pectinal nerve in the scorpion is remarkable for its size, and so, also, is the nerve to the flabellum in Limulus, while the large size of the auditory nerve in the vertebrate, in distinction to the size of the auditory apparatus, has always aroused the attention of anatomists.