situated near the junction of the caudal and trunk regions, and as a rule is median in position, rarely, as in the Dipnoi, displaced to the right or left of the middle line; but the anus differs greatly in position, sometimes retaining its primitive position at the hinder end of the trunk, as in the Holocephali, Chondrostei, Crossopterygii, Holostei, and many Teleosts, or occupying almost any position between that point and, as in the "Electric Eels" (Gymnotidae), the ventral surface of the throat (Fig. 351.) {257}[Illustration: FIG. 155.—A, alimentary canal and liver of a female _Protopterus_, from the left side. Part of the left wall of the stomach and intestine, and the peritoneal investment of the spleen have been removed. _a.p_, Abdominal pore; _b.d_, bile-duct; _b.ent_, Bursa Entiana; _cl_, cloaca; _cl.ap_, cloacal aperture; _cl.c_, caecum cloacae; _c.m.a_, coeliaco-mesenteric artery; _cy.d_, bile duct; _k.d_, kidney duct; _m.a_, mesenteric arteries; _od_, oviduct; _pt.c_, post-caval vein or inferior vena cava; _p.v_, portal vein; the other reference letters as in B. (From Newton Parker.) B, viscera of an adult female _Lepidosteus_, ventral view. The oesophagus, the commencement of the intestine and the rectum have been laid open. _ab_, air-bladder; _an_, anus; _b.d_, intestinal aperture of the bile-duct; _g.b_, gall-bladder; _gl_, oesophageal aperture of the air-bladder; _h.d_, hepatic duct; _l_, liver; _oes_, oesophagus; _py_, pylorus; _py.c_; pyloric caeca; _py.c′_, the four intestinal orifices of the pyloric caeca; _r_, rectum; _s_, spleen; _sp.v_, spiral valve; _st_, stomach. (From Balfour and Newton Parker.)] {258}[Illustration: FIG. 156.—Transverse section of a Fish, diagrammatic. _cn_, Centrum; _coel_, coelome; _d.a_, dorsal aorta; _d.f_, dorsal fin; _d.m_, dorsal muscles; _d.ms_, dorsal mesentery; _f.r_, fin ray; _gon_, gonad; _int_, intestine; _l.v_, lateral vein; _msn_, mesonephros; _msn.d_, mesonephric duct; _n.a_, neural arch; _p_, parietal layer of the peritoneum; _p′_, visceral layer; _p.c.v_, posterior cardinal vein; _pn.d_, Müllerian duct; _r_, ventral rib; _r′_, dorsal rib; _sp.c_, spinal cord; _t.p_, transverse process; _v.m_, ventral muscles; _v.ms_, ventral mesentery. (Modified, after Parker and Haswell.)]