This suggestion of Langerhans is surely most significant, considering that this somewhat separate portion of the velum, to which he assigns such a function, is in the very place where the gnathite portion of the metastomal appendages would have been situated if it were true that the lower lip and anterior portion of the velum of Ammocœtes were derived from the metastoma.
In addition to this marked serrated edge the whole surface of the anterior portion of the velum is covered over with a scale-like or tubercular pattern remarkably like the surface-ornamentation seen in many of the members of the ancient group Eurypteridæ. In Fig. [121] I give a picture of this surface-marking of the velum. It is striking to see that just as in the case of the invertebrate this marking and these serræ are formed simply by the cuticular surface of the epithelial cells; a surface which, according to Wolff, possibly contains chitin. The interpretation which I would give of the velar folds is therefore as follows:—
They represent the fused basal parts of the opercular and metastomal appendages, the gnatho-bases of the latter still retaining in a reduced degree their rasping surfaces, because, owing to their position on each side of the opening into the respiratory chamber they were still able to manipulate the food as it passed by them after the closure of the old mouth.
Fig. 120.—Ammocœtes cut open in Mid-Ventral Line to show Position of Velum; Velar Folds removed on one side.
tr., trabeculæ; vel., velum; B., anterior gnathic portion of velum; ps. br., pseudo-branchial groove; m2, muscles of lower lip segment; m3, muscles of thyro-hyoid segment; mt2, insertion of tubular muscles of velum near thyroid.
Fig. 121.—Surface View of Anterior Surface of Velum.
The whole evidence points irresistibly to the conclusion that the mandibular or velar nerve of the trigeminal does supply a splanchnic segment which is, in all respects, comparable with the segments supplied by the facial, glossopharyngeal, and vagus nerves, except that it does not possess branchiæ. This simply means that the appendages which these nerves originally supplied were prosomatic, not mesosomatic, and corresponded, therefore, to the chilarial or metastomal appendages.
A comparison of the ventral surface of Slimonia, as given in Fig. [8], p. [27], with that of Ammocœtes (Fig. [119]), when the thyroid gland and lower lip muscles have been exposed to view, enables the reader to recognize at a glance the correctness of this conclusion.