The Origin of Vertebrates

by Walter Holbrook Gaskell

But, as argued out in the last chapter, the diagram of the adult Ammocœtes must be compared with that of a cephalaspidian fish; the diagram of the palæostracan must be compared with the larval condition of Ammocœtes. In other words, Fig. [106], B, must be compared with Fig. [106], C, which represents a section through the larval Ammocœtes as it would appear if it reached the adult condition without any forward growth of the upper lip or any breaking through of the septum between the oral and respiratory chambers. The striking similarity between this diagram and that of Eurypterus becomes immediately manifest even to the smallest details. The only difference between the two, except, of course, the notochord, consists in the closure of the mouth opening (o), in Fig. [106], B, by which the olfactory passage (olf. p.) of the scorpion becomes converted into the hypophysial tube (Hy.), Fig. [106], C, and later into the nasal tube (Na.), Fig. [106], D, of the full-grown Ammocœtes. That single closure of the old mouth is absolutely all that is required to convert the Eurypterus diagram into the Ammocœtes diagram.

Such a comparison immediately explains in the simplest manner a number of anatomical peculiarities which have hitherto been among the great mysteries of the vertebrate organization. For not only do the median eyes (C.E.) correspond in position in the two diagrams, and the infundibular tube (Inf.) and the ventricles of the brain (C.C.) correspond to the œsophagus (œs.) and the cephalic stomach (Al.), as already fully discussed; but even in the very place where the narrow œsophagus opened into the wider chamber of the pharynx (Ph.), there, in all the lower vertebrates, the narrow infundibular tube opens into the wider chamber of the membranous saccus vasculosus (sac. vasc.). This is the last portion of the membranous part of the tube of the central nervous system which has not received explanation in the previous chapters, and now it is seen how simple its explanation is, how natural its presence—it represents the old pharyngeal chamber of the palæostracan ancestor.

Fig. 106.—Diagram of Sagittal Median Section through B, Eurypterus; C, Larval Ammocœtes; D, Full-grown Ammocœtes.

Next among the mysteries requiring explanation is the pituitary body, that strange glandular organ always found so closely attached to the brain in the infundibular region that when it is detached in taking out the brain it leaves the infundibular canal patent right into the IIIrd ventricle. A comparison of the two diagrams indicates that such a glandular organ (Pit.), Fig. [106], C, was there because the coxal excretory glands (cox. gl.), Fig. [106], B, were in a similar position in the palæostracan ancestor—that, indeed, the pituitary body is the descendant of the coxal glands.

Finally, the diagrams not only indicate how the mesosomatic appendage-nerves supplying in the one case the operculum and the respiratory appendages correspond to the respiratory group of nerves, VII., IX., X., supplying in the other case the thyroid, hyoid, and branchial segments, but also that a similar correspondence exists between the prosomatic appendage-nerves in the one case and the trigeminal nerve in the other; a correspondence which supplies the reason why in the vertebrate a septum originally existed between an oral and respiratory chamber.

Such a comparison, then, leads directly to the suggestion that the trigeminal nerve originally supplied the prosomatic appendages, such appendages being: 1. The metastoma, which has become in Ammocœtes the lower lip supplied by the velar or mandibular branch of the trigeminal nerve (7); 2. The ectognath, which has become the large median ventral tentacle, called by Rathke the tongue, supplied by the tongue nerve (6); 3. The endognaths, which have been reduced to tentacles and are supplied by the tentacular branch of the trigeminal nerve (2, 3, 4, 5).

I have purposely put these two diagrams of the larval Ammocœtes and of Eurypterus before the minds of my readers at this early stage of my argument, so as to make what follows more understandable. I propose now to consider fully each one of these suggestive comparisons, and to see whether or no they are in accordance with the results of modern research.

In the first instance, the diagrams suggest that the trigeminal nerve originally supplied the prosomatic appendages of the palæostracan ancestor, while the eye-muscle nerves supplied the body-muscles of the prosoma.

As these appendages did not carry any vital organs such as branchiæ, but were mainly locomotor and masticatory in function, it follows that their disappearance as such would be much more complete than that of the mesosomatic branchial appendages. Most probably, then, in the higher vertebrates no trace of such appendages might be left; consequently the segmentation due to their presence would be very obscure, so that in this region the very reverse of what is found in the region of the vagus nerve would be the rule. There branchiomeric segmentation is especially evident, owing to the persistence of the branchial part of the branchial appendages; here, owing to the disappearance of the appendages, the segmentation is no longer branchiomeric, but essentially mesomeric in consequence of the persistence of the somatic eye-muscles.