Th., thyroid gland; M., lower lip, with its muscles.
Besides the striated visceral muscles, each branchial segment possesses its own tubular muscles, shown in Fig. [116] (mt3) and (mt4). As the section shows, there is clearly a group of tubular muscle-fibres belonging to the hyoid segment (mt2), and also another group belonging to the segment in front of the hyoid (mt1); so that, judging from this section, each of these segments possesses its own tubular musculature just as do the branchial segments, the difference being that the tubular muscles are more separated from the striated visceral group than in the true branchial segments, owing to the size of the blood-spaces surrounding them. What, then, are these two groups of muscles? Tracing them in the series of sections, both groups are seen to belong to the system of velar muscles, forming an anterior and a posterior group respectively; and we see, further, that there is not the slightest trace of any tubular muscles anterior to these muscles of the velum.
In the living Ammocœtes the velar folds on each side can be seen to move synchronously with the movements of respiration, contracting at each expiration, and thus closing the slit by which the oral and respiratory chambers communicate, and so forcing the waters of respiration through the gill-slits, as described by Schneider. Such a fact is clear evidence that these tubular muscles of the velar folds belong to the same series as the tubular muscles of the branchial segments, so that if, as I have already suggested, the latter muscles were originally the veno-pericardial muscles of segments corresponding to the branchial appendages, then the former would represent the veno-pericardial muscles of the segments corresponding to the opercular and metastomal appendages. What, then, are these velar folds, and how is it that the tubular muscles of these two segments become the velar muscles? I will consider, in the first instance, the posterior group of muscles (mt2) in Fig. [116].
It has already been pointed out that the tubular muscles of the branchial segments are dorso-ventral, but do not run with the ordinary constrictors, having separate attachments and running part of their course internally to and part externally to the ordinary constrictors. At first sight, as is usually stated, the hyoid segment does not appear to possess tubular muscles at all. If, however, we follow the posterior group of velar muscles (mt2), we see (Fig. [117]) that they pass between the auditory capsule and the opercular bar (sk3) of muco-cartilage to reach the region of the jugular vein (j.v.) posteriorly to the auditory capsule, so that their dorsal origin bears the same relation to the hyoid segment as the dorsal attachment of the rest of the tubular muscles to their respective segments. Further, these muscles run along the length of the velar fold, and are attached ventrally on each side of the thyroid gland, so that their ventral attachment also corresponds in position, as regards the hyoid segment, with the ventral attachment of the rest of the tubular muscles as regards their respective segments.
This ventral attachment is shown in Fig. [119] on each side of the thyroid, and in Fig. [120] (mt2); while in Fig. [117] the fibres are seen converging to this ventral position. In other words, this large posterior muscle of the velar folds is a dorso-ventral muscle, and would actually take the same position in the hyoid segment as the dorso-ventral tubular muscles in the other branchial segments, if the velum were put back into its original position as the septum terminating the branchial chamber. Conversely, the presence of these hyoid tubular muscles in the velum gives evidence that the opercular segment takes part in the formation of the septum, as already suggested.
Miss Alcock, in her paper, speaks of tubular muscles belonging to the hyoid segment, which are attached to the muco-cartilage. Schaffer also speaks of certain tubular muscles belonging to the velar group as piercing the muco-cartilage (h. r. s.) in his figures 24 and 25, i.e. the metastomal bar, near its junction with the opercular bar. In my specimens there is a distinct group of tubular muscles which pierce the opercular bar of muco-cartilage at its junction with the metastomal bar, and pass into the posterior group of velar muscles. They clearly belong to the hyoid segment, as Miss Alcock supposed, but are not attached to the muco-cartilage. It is possible that they represent a different group to those already considered, and suggest the possibility that this opercular or thyro-hyoid segment is double with respect to its original veno-pericardial muscles as well as in other respects.
The anterior group of tubular muscles (mt1, Figs. 116, 117) belonging to the same segment as the metastomal bar must now be taken into consideration. Very different is their origin to that of the posterior group: they arise close up against the eye, and have given rise to Kupffer's and Hatschek's misconception that the superior oblique muscle of the eye arises from a part of the velar musculature. Naturally, as Neal has pointed out, they have nothing to do with the eye-muscles; the superior oblique muscle is plainly in its true place entirely apart from these velar muscles, which form the foremost group of the segmental tubular muscles. They pass into the anterior part of the velar folds and run round to the ventral side just in the same way as does the posterior group. This anterior group of tubular muscles represents the veno-pericardial muscle of the segment immediately in front of the opercular, i.e. the metastomal segment, and is the foremost of these veno-pericardial muscles. Its presence shows that the velar folds, formed as they were by the breaking down of the septum, are in reality part of two segments, viz. the opercular and the metastomal, which have fused together in their basal parts, and by such fusion have caused the inter-relationship between the VIIth and Vth nerves, so apparent in the anatomy of the vertebrate cranial nerves.
A further piece of evidence that this anterior portion of the velum belongs to the same segment as the lower lip is the fact that in addition to the tubular muscles a single ordinary striated muscle is found in the velum which, like the muscles of the lower lip, is innervated by this same mandibular nerve.
This muscle is attached laterally to the muco-cartilage of the metastomal bar (sk2) at its junction with the muco-cartilage of the lower lip, and spreads out into a number of strands which are attached at intervals along the whole length of the free anterior edge of the velum. It is the only non-tubular muscle belonging to the velum, and by its contraction it draws the anterior portions of the velar folds apart from each other, and so opens the slit between them, through which the food and mud must pass. Clearly from its position it does not belong to the original tergo-coxal group of muscles as do those of the lower lip; it must have been one of the intrinsic muscles of the metastoma itself.
This anterior portion of the velar folds affords yet another striking hint of the correctness of my comparison of the lower lip segment of Ammocœtes 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.