The Origin of Vertebrates

by Walter Holbrook Gaskell

The logical conclusion is that the appendages of the Palæostraca possessed special sense-organs concerned with the perception of special vibrations, especially in the mesosomatic or respiratory region, and that somewhere at the junction of the prosoma and mesosoma, one of these sense-organs was specially developed to form the origin of the vertebrate auditory apparatus.

Impressed by this reasoning I made search for some specially striking sense-organ at the base of one of the appendages of Limulus, at the junction of the prosoma and mesosoma, and was immediately rewarded by the discovery of the extraordinary nature of the flabellum, which revealed itself as an elaborate sense-organ supplied with a nerve out of all proportion to its size. Up to this time no one had the slightest conception that this flabellum was a special sense-organ; the discovery of its nature was entirely due to the logical following out of the theory of the origin of vertebrates described in this book.

The structure of this large sense-organ is comparable with that of the sense-organs of the pectens of the scorpion, and of many other organs found on the appendages of various members of the scorpion group, of arachnids and other air-breathing arthropods. Many of these organs, such as the lyriform organs of arachnids, and the 'halteres' or balancers of the Diptera, are usually regarded as auditory and equilibration organs.

On all the mesosomatic appendages of Limulus very remarkable sense-organs are found, apparently for estimating pressures, which, when the appendages sank into the body to form with their basal parts the branchial diaphragms of Ammocœtes, could easily be conceived as remaining at the surface, and so giving rise to the lateral line organs.

Further confirmation of the view that an organ, such as the flabellum, must be looked upon as the originator of the vertebrate auditory organ, is afforded by the extraordinary coincidence that in Limulus a diverticulum of the generative and hepatic mass accompanies the flabellar nerve into the basal part of the digging appendage, while in Ammocœtes, accompanying the auditory nerve into the auditory capsule, there is seen a mass of cells belonging to that peculiar tissue which fills up the space between the brain and the cranial walls, and has already, on other grounds, been homologized with the generative and hepatic masses which fill up the encephalic region of Limulus.

For all these reasons special sense-organs, such as are found in the flabellum of Limulus and in the pectens of scorpions, may be looked upon as giving origin to the vertebrate auditory apparatus. In such case it is highly probable that the parachordals, with the auditory capsules attached, arose from a second entochondrite of the same nature as the plastron; a probability which is increased by the fact that the scorpion does possess a second entochondrite, which, owing to its special relations to the pecten, is known as the supra-pectinal entochondrite.

CHAPTER XII

THE REGION OF THE SPINAL CORD

Difference between cranial and spinal regions.—Absence of lateral root.—Meristic variation.—Segmentation of cœlom.—Segmental excretory organs.—Development of nephric organs; pronephric, mesonephric, metanephric.—Excretory organs of Amphioxus.—Solenocytes.—Excretory organs of Branchipus and of Peripatus, appendicular and somatic.—Comparison of cœlom of Peripatus and of vertebrate.—Pronephric organs compared to coxal glands.—Origin of vertebrate body-cavity (metacœle).—Segmental duct.—Summary of formation of excretory organs.—Origin of somatic trunk-musculature.—Atrial cavity of Amphioxus.—Pleural folds.—Ventral growth of pleural folds and somatic musculature.—Pleural folds of Cephalaspidæ and of Trilobita.—Significance of the ductless glands.—Alteration in structure of excretory organs which have lost their duct in vertebrates and in invertebrates.—Formation of lymphatic glands.—Segmental coxal glands of arthropods and of vertebrates.—Origin of adrenals, pituitary body, thymus, tonsils, thyroid, and other ductless glands.—Summary.

The consideration of the auditory nerve and the auditory apparatus terminates the comparison between the cranial nerves of the vertebrate and the prosomatic and mesosomatic nerves of the arthropod, and leaves us now free to pass on to the consideration of the vertebrate spinal nerves and the organs they supply. Before doing so, it is advisable to pass in review the conclusions already attained.