The Origin of Vertebrates

by Walter Holbrook Gaskell

Fig. 155.—A, The Digging Appendage or Ectognath of Limulus; B, The Middle Protuberance (2) of the Entocoxite opened, to show the Generative and Hepatic Tissue (gen.) within it; C, One of the Prosomatic Locomotor Appendages or Endognaths of Limulus, for comparison with A.

fl., flabellum; cox., coxopodite; ent., entocoxite; m., mandible; i.m., inner mandible or epicoxite.

Patten and Redenbaugh, in their description of the prosomatic appendages of Limulus, describe the segments of the limbs as (1) the dactylopodite, (2) the propodite, (3) the mero- and carpo-podites, (4) the ischiopodite, (5) the basipodite, and (6) the coxopodite (cox. in Fig. [155]). Still more basal than the coxopodite is situated the entocoxite (ent. in Fig. [155]), which is composed of three sclerites or sensory knobs, to use Patten's description. The middle one of these three sclerites enlarges greatly in the digging appendage, and grows over the coxopodite to form the base from which the flabellum springs. Thus, as they have pointed out, the flabellum does not belong to the coxopodite of the appendage, but to the middle sensory knob of the entocoxite. Upon opening the prosomatic carapace, it is seen that the cephalic generative and hepatic masses press closely against the internal surface of the prosomatic carapace and also of the entocoxite, so that any enlargement of one of the sensory knobs of the entocoxite would necessarily be filled with a protrusion of the generative and hepatic masses. This is the reason why the generative and hepatic material apparently passes into the basal segment of the ectognath, and not into that of the endognaths; it does not really pass into the coxopodite of the appendage, but into an enlarged portion of the entocoxite, which can hardly be considered as truly belonging to the appendage. Kishinouye has stated that a knob arises in the embryo at the base of each of the prosomatic locomotor appendages, but that this knob develops only in the last or digging appendage (ectognath) forming the flabellum. Doubtless the median sclerites of the entocoxites of the endognaths represent Kishinouye's undeveloped knobs.

I conclude, therefore, that the flabellum, together with its basal part, is an adjunct to the appendage rather than a part of it, and might, therefore, easily remain as a separate and well-developed entity, even although the appendage itself dwindled down to a mere tentacle.

The evidence appears to me very strong that the flabellum of Limulus and the pecten of scorpions are the most likely organs to give a clue to the origin of the auditory apparatus of vertebrates. At present both the Eurypterids and Cephalaspids have left us in the lurch; in the former there is no sign of either flabellum or pecten; in the latter, no sign of any auditory capsule beyond Rohon's discovery of two small apertures situated dorsally on each side of the middle line in Tremataspis, which he considers to be the termination of the ductus endolymphaticus on each side. In both cases it is probable, one might almost say certain, that any such special sense-organ, if present, was not situated externally, but was sunk below the surface as in Ammocœtes.

The method by which such a sense-organ, situated externally on the surface of the animal, comes phylogenetically to form the lining wall of an internally situated membranous capsule is given by the ontogeny of this capsule, which shows step by step how the sense-organ sinks in and forms a capsule, and finally is entirely removed from the surface except as regards the ductus endolymphaticus.

Summary.

The special apparatus for hearing is of a very different character from that for vision or for smell, for its nerve belongs to the infra-infundibular group of nerves, and not to the supra-infundibular, as do those of the other two special senses. Of the five special senses the nerves for touch, taste, and hearing, all belong to the infra-infundibular segmental nerve-groups. The invertebrate origin, then, of the vertebrate auditory nerve must be sought for in the infra-œsophageal segmental group of nerves, and not in the supra-œsophageal.

The organs supplied by the auditory nerve are only partly for the purpose of hearing; there is always present also an apparatus—the semicircular canals—concerned with equilibration and co-ordination of movements. Such equilibration organs are not confined to the auditory nerve, but in the water-living vertebrates are arranged segmentally along the body, forming the organs of the lateral line in fishes; the auditory organ is but one of these lateral line organs, which has been specially developed.

These lateral line organs have been compared to similar segmental organs found in connection with the appendages in worms, especially the respiratory appendages. In accordance with this suggestion we see that they are all innervated from the region of the respiratory nerves—the vagus, glosso-pharyngeal, and facial—nerves which originally supplied the respiratory appendages of the palæostracan ancestor.