The Origin of Vertebrates

by Walter Holbrook Gaskell

A-B, the line of epibranchial ganglia; au., auditory capsule; nc., notochord; Hy., tube of hypophysis; Or., oral cavity; u.l., upper lip; l.l. lower lip; vel., septum between oral and respiratory cavities; V., VII., IX., X., cranial nerves; x., nerve with four epibranchial ganglia.

In accordance with the views put forward in this book, a possible interpretation of these plakodal ganglia would be given as follows:—

Beard, who, after Froriep, drew attention to this relation of the cranial ganglia to special skin-patches, has compared them with the parapodial ganglia of annelids, i.e. ganglia in connection with annelidan appendages; whether we are here obtaining a glimpse of the far-off annelidan ancestry of both arthropods and vertebrates it would be premature at present to say. It is natural enough to expect, on my view, to find evidence of annelidan ancestry in vertebrate embryology (as has been so often asserted to be the case), seeing that undoubtedly the Arthropoda are an advanced stage of Annelida; and, indeed, the way is not a long one when we consider Beecher's evidence that the Trilobita belong to the Phyllopoda, certainly a primitive crustacean group, which Bernard derives directly from the annelid group Chætopoda. If, then, these plakodal ganglia indicate the former presence of appendages, we obtain this result:—The foremost ganglion on each side possesses one plakodal ganglion, and therefore indicates an anterior pair of appendages, possibly the cheliceræ. Then comes the peculiar nerve with four plakodal ganglia indicating on each side four appendages close together, possibly the endognaths. Then, finally, on each side, the second large ganglion with two plakodal ganglia, indicating two pairs of appendages, possibly the ectognaths and the metastoma.

Summary.

The consideration of the history of the cranial segmentation shows that whereas, from the commencement of that history, the evidence for two ventral segments supplied by the trigeminal nerve is clear and unmistakable, later observers have tended more and more to increase the number of these segments, until at the present time the evidence is in favour of at least six, probably seven, as the number of segments supplied by the motor part of the trigeminal.

So, also, the original evidence for the number of dorsal or somatic segments limits the number to three, innervated respectively by the oculomotor (III.), trochlear (IV.), and abducens (VI.) nerves, or rather two, since the last nerve belongs to the facial segment. The muscles which these three nerves supply are derived respectively from the walls of the premandibular, mandibular, and hyoid cœlomic cavities.

Later evidence points strongly to the conclusion that the oculomotor nerve and the premandibular cavity represent not one segment but the fusion of four, while the mandibular cavity represents two segments. In addition to these, Miss Platt has discovered a still more anterior head-cavity, which she has named the anterior cavity, so that the pro-otic segments on this reckoning are seven in number, viz.: (1) the anterior cavity, (2, 3, 4, 5) the premandibular cavity, (6, 7) the mandibular cavity. The somatic muscles belonging to these dorsal segments are the eye-muscles, which are all dorso-ventral in position, and are not the same as the longitudinal somatic muscles, but belong to a distinct dorso-ventral segmental group, the only representative of which at present known in the mesosomatic region is the external rectus innervated by the VIth nerve.

These head-cavities, and these muscles of the vertebrate, resemble the corresponding cavities and muscles of the invertebrate to an extraordinary degree, so that it becomes easy to see how the dorso-ventral muscles of the prosomatic segments of the latter have become converted into the eye-musculature of the former. The most powerful proof of all that such a conversion has taken place is that a natural and simple explanation is at once given of the extraordinary course taken by the IVth or trochlear nerve. Ever since neurology began, the course of this nerve has arrested the attention of anatomists. Why should just this one pair of nerve-roots of all those in the whole body be directed dorsalwards instead of ventralwards, and cross each other in the valve of Vieussens, each to supply a simple eye-muscle (the superior oblique) belonging to the other side? For generations anatomists have wondered and found no solution, and yet, without any straining of hypotheses, in consequence simply of the investigation of the anatomy of the corresponding pair of muscles in the scorpion group, the solution is immediately apparent.

This pair of muscles alone, of all the musculature attached to the carapace, crosses the mid-dorsal line to be attached to the other side, thus carrying its nerve with it to the other side; by a continuation of the same process the relation of the trochlear to the superior oblique muscle can be explained.

The comparison of the eye-muscles of the vertebrate with the dorso-ventral segmented muscles of the invertebrate makes the number and nature of the pro-otic segments much clearer.