The bar which comes first for consideration (sk3) arises immediately behind the auditory capsule from the first branchial cartilage very soon after it leaves the sub-chordal cartilaginous ligament; the soft cartilage of the sub-chordal ligament ceases abruptly in its extension along the notochord at the place where the hard cartilage of the parachordal joins it, and in a sense it may be said to leave the notochord at this place and pass into the basal part of the first branchial bar. The most anterior continuation of this branchial system is this muco-cartilaginous bar (sk3), which passes forward and ventralwards, being separated from the axial line by the auditory capsule (cf. Fig. 118, A, B, C). Its position is well seen in a sagittal section, such as Fig. [117]. It follows absolutely the line of the pseudo-branchial groove (ps. br., Fig. [114]), and ventrally joins the plate of muco-cartilage which covers the thyroid gland. It forms a thickened border to this plate anteriorly, just as the branchial cartilaginous bars border it posteriorly. In fact, it behaves with respect to the hyoid segment in a manner similar to the rest of the cartilaginous bars with respect to their respective segments.
It represents, although composed of muco-cartilage, the cartilaginous bar of the operculum in Limulus, which also forms the termination of the branchial cartilaginous system, as fully explained in Chapter III.; it may therefore be called the opercular bar.
The next bar (sk2) is extremely interesting, as we are now out of the branchial or mesosomatic region, and into the region corresponding to the prosoma. It starts from a cartilaginous projection made of hard cartilage, just in front of the auditory capsule, called by Parker the 'pedicle of the pterygoid'—a projection (ped.) which defines the posterior limit of the trabeculæ on each side, where they join on to the parachordals,—and winding round and below the auditory capsule, joins the opercular bar (cf. Fig. [118]), to pass thence into and form part of the muco-cartilaginous plate of the lower lip. In the section figured (Fig. [116]), this projection of hard cartilage is not directly continuous with (sk2), owing to a slight curvature in the bar; the next few sections show clearly the connection between (ped.) and (sk2), and consequently the complete separation by means of this bar of the hyoid segment from the segment in front.
Fig. 118.—Skeleton of Head-Region of Ammocœtes. A, Lateral View; B, Ventral View; C, Dorsal View.
Muco-cartilage, red; soft cartilage, blue; hard cartilage, purple. sk1, sk2, sk3, skeletal bars; c.e., position of pineal eye; na. cart., nasal cartilage; ped., pedicle; cr., cranium; nc., notochord.
In the figures, the hard cartilage is coloured purple, the soft cartilage blue, and the muco-cartilage red, so that the position of this bar is well shown. This bar may be looked upon as bearing the same relation to the muco-cartilaginous plate of the lower lip as the opercular bar does to the muco-cartilaginous plate over the thyroid; and seeing that these two plates form one continuous ventral head-shield of muco-cartilage (Fig. [118], B), and also that this bar fuses with the opercular bar, we may conclude that the segment represented by the lower lip is closely connected with the hyoid or opercular segments. In other words, if the lower lip arose from the metastoma, then this pair of skeletal bars might be called the metastomal bars, which formed the supporting skeleton of the last pair of prosomatic appendages and, as is likely enough, arose in connection with the posterior lateral horns of the plastron; these posterior lateral horns, like the rest of the plastron, would give rise to hard cartilage, and so form in Ammocœtes the two lateral so-called pterygoid projections.
In the branchial region the muscles which marked out each branchial segment were of two kinds—ordinary striated visceral muscles and tubular muscles. Of these the former represented the dorso-ventral muscles of the branchial appendages, while the latter formed a separate group of dorso-ventral muscles with a separate innervation which may have been originally the segmental veno-pericardial muscles so characteristic of Limulus and the scorpions. In Figs. 116, 117, the grouping of these muscles in each branchial segment is well shown, and it is immediately seen that the hyoid segment possesses its group of striated visceral muscles (m3) supplied by the VIIth nerve in the same manner as the posterior groups, as has already been pointed out by Miss Alcock in her previous paper. Passing to the segment in front, Fig. [116] shows that the group of visceral muscles (m2) corresponds in relative position with respect to the metastomal bar to the hyoid muscles with respect to the opercular bar or to the branchial visceral muscles with respect to each branchial bar. What, then, is this muscular group? The series of sections show that these are the dorso-ventral muscles belonging to the lower lip, which, as seen in Fig. [119] (M.), form a well-marked muscular sheet, whose fibres interlace across the mid-ventral line of the lower lip. This group of lower lip-muscles is very suggestive, for these muscles arise, not from the trabeculæ, but from the front dorsal region of the cranium, just in front of the two lateral eyes. In Fig. [117] the dorsal part is seen cut across on its way to its dorsal attachment. Such an origin is reminiscent of the tergo-coxal group of muscles, arising, as they do, from the primordial cranium and the tergal carapace, and suggests at once that when the chilarial appendages expanded to form a metastoma, their tergo-coxal muscles formed a sheet of muscles similar to those of the lower lip of Ammocœtes, by which the movements of the metastoma were effected. The posterior limit of these muscles ventrally marks out the junction of the segment of the lower lip with that of the thyroid; in other words, indicates where the metastoma had fused ventrally with the operculum (Fig. [117]).
Fig. 119.—Ventral View Of Head-Region of Ammocœtes.