The Origin of Vertebrates

by Walter Holbrook Gaskell

Summary.

The general consideration of the evidence of the number of segments, and their nature in the pro-otic region of the vertebrate, as given in the last chapter, is not incompatible with the view that the trigeminal nerve originally supplied seven appendages, which appendages did not carry branchiæ, but were originally used for purposes of locomotion as well as of mastication.

Such appendages clearly no longer exist in the higher vertebrates, the muscles of mastication only remaining; but in the earliest fish-forms they must have existed, as, indeed, is seen in Pterichthys and Bothriolepis. Judging from all the previous evidence some signs of their existence may reasonably be expected still to remain in Ammocœtes. Such is indeed the case.

In the adult Petromyzon the trigeminal nerve innervates specially a massive suctorial apparatus, by means of which it holds on to other fishes, or to stones in the bottom of the stream. There is here no apparent sign of appendages. Very great, however, is the difference in the oral chamber of Ammocœtes; here there is no sign of any suctorial apparatus, but instead, a system of tentacles, together with the remains of the septum or velum, which originally closed off the oral from the respiratory chamber. These tentacles are the last remnants of the original foremost prosomatic appendages of the palæostracan ancestor. Like the lateral eyes they do not develop until the transformation comes, but during the whole larval condition their musculature remains in an embryonic condition, and then from these embryonic muscles the whole massive musculature of the suctorial apparatus develops; a sucking apparatus derived from the modification of appendages, as so frequently occurs in the arthropods.

The study of Ammocœtes indicates that the velum and lower lip correspond to the metastoma of the Eurypterid, i.e. the chilaria of Limulus, while the large ventral pair of tentacles, called the tongue, correspond to the ectognaths of the Eurypterids, and probably to the oar-like appendages of Pterichthys and Bothriolepis. From these two splanchnic segments the suctorial apparatus in the main arises; the motor supply of these two segments forms the mass of the trigeminal nerve-supply, and the nerves supplying them, the velar nerve and the tongue-nerve, are markedly separate from the rest of the trigeminal nerve.

The rest of the tentacles present much less the sign of independent segments. In their nerves, their muco-cartilaginous skeleton, and their rudimentary muscles, they indicate a concentration and amalgamation, such as might be expected from the concentrated endognaths. The continuation of the dwindling process, already initiated in the Eurypterid, would easily result in the tentacles of Ammocœtes.

The nasal tube of Ammocœtes, which originates in the hypophysial tube, corresponds absolutely in position and in its original structure, to the olfactory tube of a scorpion-like animal. From this homology two conclusions of importance follow: (1) the old mouth, or palæostoma, of the vertebrate was situated at the end of this tube, therefore, at the termination of the infundibulum; (2) the upper lip, which by its growth, brings the olfactory tube from a ventral to a dorsal position, was originally formed by the foremost sternites or endostoma, or else by the sterno-coxal processes of the second pair of prosomatic appendages of the palæostracan ancestor.

In strict accordance with the rest of the comparisons made in this region, the pituitary body shows by similarity of structure, as well as of position, that it arose from the coxal glands, which were situated at the base of the four endognaths.

One after another, when once the clue has been found, all these mysterious organs of the vertebrate, such as the pituitary and thyroid glands, fall harmoniously into their place as the remnants of corresponding important organs in the palæostraca.

Yet another clue is afforded by the tubular muscles of Ammocœtes, that strange set of non-vertebrate striated muscles, which are so markedly arranged in a segmental manner, which disappear at transformation, and are never found in any of the higher vertebrates, for the limits of their distribution correspond to the veno-pericardial muscles of Limulus.