The branchial lamellae are represented by a series of vascular horizontal and parallel ridges radiating outwards along the roof, floor, and lateral walls of each gill-sac, and invested by an {281}epithelium which is partially ciliated. The inter-branchial septa are much thicker than in Elasmobranchs, and include not only the walls of adjacent sacs and the branchial muscles, but also contain cavernous peribranchial lymph-sinuses. The cartilaginous branchial skeleton is situated wholly external to the gill-sacs, the so-called branchial arches lying between the external apertures of the sacs, and directly beneath the superficial skin, or, in other words, on the outer margins of the inter-branchial septa, and not on the inner, as is invariably the case with the branchial arches of Fishes. [Illustration: FIG. 163—Dissection of _Myxine glutinosa_ from the left side. _au.c_, Auditory capsule; _br.ap_, left branchial aperture; _br.b_, rudiment of branchial basket; _br.s.1_, first gill-sac; _c.br.t_, common branchial tube; _cn.c_, cornual cartilage; _gul_, gullet; _ht_, heart; _lg.m_, lingual muscles; _m.v.c_, median ventral cartilage; _na.t_, nasal tube; _nch_, notochord; _n.t_, neural tube; _oes.ct.d_, oesophageo-cutaneous duct; _p.l.c_, posterior lateral cartilage; _sb.oc.a_, subocular arch; _sp.c_, spinal cord; _st.p_, styloid process. (After W. K. Parker, from Parker and Haswell's _Zoology_.)] In the Hag-Fish (_Myxine_) (Fig. 163), there are usually six, very rarely seven, pairs of gill-sacs, all of which open directly into the pharynx, and not into a branchial canal as in the Lampreys. On the other hand, _Myxine_ is unique in having the outer extremities of its gill-sacs produced into a corresponding number of tubular canals which, after a longer or shorter course obliquely backwards and outwards, unite to form on each side a ventrally-situated external aperture (Fig. 163). In the same genus a short canal, or oesophageo-cutaneous duct, passes from the pharynx {282}behind the last gill-sac of the left side, and opens externally with the common external branchial aperture of that side. In _Bdellostoma_ there are usually six or seven pairs of gill-sacs, but some species have ten or even fourteen pairs.[283] They agree with those of the Lamprey in having independent external apertures, but resemble the corresponding organs in _Myxine_ in opening directly into the pharynx. An oesophageo-cutaneous duct is also present.[284]