_Tr._, trabeculæ; _Pit._, pituitary space; _Inf._, infundibulum; _Ser._, median serrated flange of velar folds.] {293}[Illustration: FIG. 116.--HORIZONTAL SECTION THROUGH THE ANTERIOR PART OF AMMOCOETES, IMMEDIATELY VENTRALLY TO THE AUDITORY CAPSULE. _sk_1_-_sk_5_, skeletal bars; _m_1_-_m_5_, striated visceral muscles; _mt_1_-_mt_4_, tubular muscles; _br_1_-_br_3_, branchiæ; _tr._, trabeculæ; _inf._, infundibulum; _ped._, pedicle; _V._, trigeminal nerve. Muco-cartilage, _red_; soft cartilage, blue; hard cartilage, purple.] {294}[Illustration: FIG. 117.--SAGITTAL LATERAL SECTION THROUGH THE ANTERIOR PART OF AMMOCOETES. Lettering and colouring same as in Fig. 116. _aud._, auditory capsule; _j.v._, jugular vein.] In Chapter IV., Figs. 63, 64, I have given a representation of the ventral and dorsal views of an Ammocoetes cut in half horizontally. Such a section shows with great clearness the series of branchial appendages with their segmental muscles and cartilaginous bars which form the branchial segments innervated by the IXth and Xth nerves, according to my view of the branchial unit. As is seen (Fig. 64 or 115), the skeletal bar of the hyoid or opercular appendage, which is clearly serially homologous with the other branchial bars, is composed of muco-cartilage, and not of cartilage. If we follow this series of horizontal sections nearer to the origin of the cartilaginous bars from the sub-chordal cartilaginous rod on each side of the notochord, we obtain a picture, as in Fig. 116, in which each branchial segment is defined by the section of the branchial cartilaginous bar (_sk_4_, _sk_5_), by the section of the separate branchiæ (_br_2_, _br_3_), and by the separate segmental muscles arranged round each bar, these muscles being partly ordinary striated (_m_4_, _m_5_), partly tubular (_mt_3_, _mt_4_). The uppermost of these branchial segments shows the same arrangement; (_sk_3_) is the branchial skeletal bar, which is now composed of muco-cartilage, not cartilage; (_br_1_) is the branchiæ in the same situation as the others, but here composed of glandular rather than of respiratory epithelium, while the ordinary striated branchial muscles of this segment are marked as (_m_3_), being separated from the tubular muscles of the segment (_mt_2_), owing to the large size of the blood-space in which these latter muscles are lying. In front of this segment so defined we see again another well-marked skeletal bar (_sk_2_) of muco-cartilage, evidently indicating a similar segment anterior to the hyoid segment. In connection with this bar there are no branchiæ, but again we see two sets of visceral muscles, the one ordinary striated, marked (_m_2_), and the other tubular, marked (_mt_1_). Here, then, the section indicates the existence of a segment of the same character as the posteriorly situated branchial segments but belonging to a non-branchial region--a segment which would represent a non-branchial appendage, the