In all the Cyclostomata the body is Eel-like in shape, the head and trunk being nearly cylindrical, and the tail somewhat flattened from side to side. In _Petromyzon_ the head terminates in a ventrally-directed, funnel-like cavity—the buccal funnel—in the roof of which the relatively small mouth is situated (Fig. 91, A.). The margin of the funnel is fringed by a series of short papillae, {151}but in the Hag-Fishes (_Myxine_ and _Bdellostoma_), where a buccal funnel is not developed, longer tentacle-like structures are present on each side of the mouth. On the upper surface of the head is the single median nostril, or naso-pituitary aperture, placed between the eyes in the Lampreys (Fig. 91, B, C), but at the anterior margin of the head in _Myxine_ and its allies (Fig. 92). In the living Lampreys a semi-transparent area of skin may be noticed behind the nasal organ, which coincides with the position of the more deeply-seated parietal eye. On each side of the body, commencing a short distance behind the eye, is a series of small and almost circular branchial clefts (_Petromyzon_, _Bdellostoma_). In _Myxine_, however, the clefts of each side have a single common external aperture, situated on the ventral side of the body and some distance behind the head (Fig. 92, A). At the junction of the trunk with the tail is the anus, behind which is the papilla which carries the urino-genital aperture at its extremity. There are no paired limbs or vestiges of such organs. Median fins are represented in the Lampreys by an anterior dorsal fin and a posterior dorsal fin, the latter being continuous with the caudal fin which fringes the upper and lower margins of the protocercal tail. In _Myxine_ a caudal fin only is present, surrounding the extremity of the tail. [Illustration: FIG. 92.—Head of _Myxine glutinosa_ (A), and of _Bdellostoma forsteri_ (B), from beneath. _br.ap_, Left external branchial aperture; _br.cl.1_, first branchial cleft; _mth_, mouth; _na.ap_, nasal aperture; _oes.ct.d_, oesophageo-cutaneous duct. The smaller openings in A are those of mucous glands. (From Parker and Haswell, after W. K. Parker.)] {152}[Illustration: FIG. 93.—_Tilapia dolloi_. To show the external characters of an Acanthopterygian Teleost. A, side view; B, the first branchial arch. _a.f_, Spinose part of the anal fin; _a.f^1_, soft rays; _c.f_, caudal fin; _d.f_, spinose portion of the dorsal fin; _d.f^1_, soft rays; _g.f_, gill filaments; _g.r_, gill rakers; _i.l.l_, inferior lateral line; _n_, nostril; _p.f_, pelvic fin; _p.op_, preoperculum; _pt.f_, pectoral fin; _s.l.l_, superior lateral line; _t.s_, transverse row of scales. (From Boulenger.)]