last, therefore, of the prosomatic appendages. Let us, then, follow {295}out these two segmental muco-cartilaginous bars and their attendant muscles, and see to what sort of segments their investigation leads. The bar which comes first for consideration (_sk_3_) 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 (_sk_3_), 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 (_sk_2_) 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 (_sk_2_), owing to a slight curvature in the bar; the next few sections show clearly the connection between (_ped._) and (_sk_2_), and consequently the complete separation by means of this bar of the hyoid segment from the segment in front. {296}[Illustration: FIG. 118.--SKELETON OF HEAD-REGION OF AMMOCOETES. A, LATERAL VIEW; B, VENTRAL VIEW; C, DORSAL VIEW. Muco-cartilage, _red_; soft cartilage, _blue_; hard cartilage, _purple_. _sk_1_, _sk_2_, _sk_3_, skeletal bars; _c.e._, position of pineal eye; _na. cart._, nasal cartilage; _ped._, pedicle; _cr._, cranium; _nc._, notochord.]