{255}[Illustration: FIG. 154.—Dissection of a male Teleost (_Salmo fario_) from the left side. _a.bl_, Air-bladder opened; _an_, anus; _au_, auricle; _b.a_, bulbus aortae; B.HY, basi-hyal; B.OC, basioccipital; _cd.a_, caudal artery; _cd.v_, caudal vein; CN, centrum; _crb_, cerebellum; _d.f.1_, first dorsal fin; _D.F.R_, dermal fin-rays; _du_, duodenum or anterior segment of the intestine; _FR_, frontal; _g.bl_, gall-bladder; _gul_, oesophagus or gullet; H.SP, haemal spine; _int_, intestine; _kd_, kidney; _kd′_, "head-kidney"; _lg_, tongue; _lr_, liver; N.SP, neural spine; _opt.l_, optic lobes; _PA.SPH_, parasphenoid; _ph_, pharynx; _pn.b_, pineal body; _pn.d_, bristle passed into ductus pneumaticus; _prsen_, prosencephalon; _pty.b_, pituitary body; PTG, pterygiophores, or radial elements of dorsal and ventral fins; _pv.f_, pelvic fin; _py.c_, pyloric caeca; _S.ETH_, supra-ethmoid; S.OC, supra-occipital; _spl_, spleen; _st_, stomach; _ts_, testis; _u.bl_, urinary bladder; _u.g.s_, urino-genital sinus and its external aperture; _ur_, ureter or kidney-duct; _v_, ventricle; _v.ao_, ventral aorta; _v.df_, vas deferens; _v.f_, ventral fin; _VO_, vomer. (From Parker and Haswell.)] {256}In the remaining Fishes the degree of convolution varies within rather wide limits. The oesophagus is usually straight and wide, but in _Lutodeira_, among Teleosts, it is long and even convoluted, and in the Plectognath Teleosts it gives off a large sac-like outgrowth ("air-sac"), which extends anteriorly as far as the head, and posteriorly to the beginning of the tail, and communicates with the oesophagus by two apertures. The stomach may be U-shaped with the concavity directed forwards, and consisting of a right limb passing backwards from the oesophagus, and a left limb curving forwards to its junction with the intestine (Fig. 153). In such instances as these the stomach and the adjacent section of the intestine describe a characteristic siphonal curve. In certain other Fishes (Fig. 160), the oesophageal portion of the stomach terminates behind in a tubular or sac-like dilatation at some distance posterior to the laterally situated pylorus, which indicates the origin of the intestine. The intestine is straight, or nearly so, in Elasmobranchs, Crossopterygii, and Dipnoi, and also in a few Teleosts; but sometimes, and very generally in Teleosts, it is more or less convoluted, notably in some of the Mugilidae, and in the Loricariidae, where, as in _Plecostomus_, it is disposed in numerous spiral coils like a watch-spring. The terminal portion of the intestine or rectum either opens into a cloaca, which also receives the urinary and genital ducts, as in Elasmobranchs (Fig. 153), and Dipnoi (Fig. 155, A), or opens externally by an anus, situated in front of the separate or united urinogenital ducts, as is the case with all the remaining groups of Fishes (Fig. 154). The cloacal aperture is invariably