This anterior portion of the velar folds affords yet another striking hint of the correctness of my comparison of the lower lip segment of Ammocoetes with the chilaria of Limulus or the metastoma of Eurypterus; for the most dorsal anterior portion, which at its attachment possesses a wedge of muco-cartilage, forms a separate, well-defined, rounded basal projection marked _Ser._ in Fig. 115, and _B_ in the accompanying Fig. 120. This is that part of the velar folds which comes together in the middle line and closes the entrance into the respiratory chamber. The epithelial surface here is most striking and suggestive, for it is markedly serrated, being covered with a large number of closely-set projections or serræ. The serration of the surface here is of so marked a character that Langerhans considered this part of the velar folds to act as a masticating organ, grinding and rasping the food and mud which passed through the narrow slit. In fact, Langerhans supposed that this portion of the velum acted in a manner closely resembling the action of the gnatho-bases of the prosomatic appendages in Limulus or the Eurypteridæ. 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 Ammocoetes were derived from the metastoma. In addition to this marked serrated edge the whole surface of {302}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. [Illustration: FIG. 120.--AMMOCOETES 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; _m_2_, muscles of lower lip segment; _m_3_, muscles of thyro-hyoid segment; _mt_2_, insertion of tubular muscles of velum near thyroid.] [Illustration: FIG. 121.--SURFACE VIEW OF ANTERIOR SURFACE OF VELUM.]