Directly behind the head in Elasmobranchs, or beneath its hinder part in all other Fishes, are placed the external apertures of the branchial clefts. In the former group these apertures are visible externally in the form of a series of narrow vertical slits, but in the latter they communicate with the exterior by opening on each side into a common branchial cavity, the outer wall of which is formed by a movable flap-like fold with a free hinder margin and a special internal skeleton of cartilaginous rays or of bony plates and rods, the gill-cover or operculum (Fig. 161, B). Behind the free margin of the operculum there is a slit-like orifice, the gill-opening or external branchial aperture, which curves from above downward and forward toward the chin, and places the branchial cavity in communication with the exterior. Through this aperture the water, which has entered through the mouth, traversed the gill-clefts, and bathed the gills, finds its exit from the body. The space on the ventral side of the head between the two halves of the lower jaw, and between the two external branchial apertures, is termed the "isthmus." The size of the external branchial aperture differs greatly in different Fishes, according to the extent to which the {156}free opercular margin fuses below with the isthmus, or behind with the side of the head. Thus the aperture may extend from the chin in front upward and backward to near the dorsal surface of the head, or it may be reduced to little more than a mere pore situated on any part of the opercular edge (e.g. _Hippocampus_); or, as in _Symbranchus_, the reduced pores of opposite sides may coalesce in the floor of the throat in a common median opening. In the Elasmobranchs and in the Dipnoi the cloacal aperture is always situated at the junction of the trunk with the tail. In the Teleostomi, however, where the intestine has a separate external orifice or anus, distinct from, and placed in front of, the separate or combined urino-genital ducts, the anus may either retain its primitive position near the union of the trunk and tail, or occupy almost any intermediate position between this point and the throat.