In the majority of fishes the _Nostrils_ are a double opening on each side of the upper surface of the snout; the openings of each side being more or less close together. They lead into a shallow groove; and only in one family (the Myxinoids) perforate the palate. In this family, as well as in the Lampreys, the nasal aperture is single. In many Eels the openings are lateral, the lower perforating the upper lip. In the Sharks and Rays (Fig. 1, p. 34) they are at the lower surface of the snout, and more or less confluent; and, finally, in the Dipnoi and other Ganoids, one at least is within the labial boundary of the mouth. The space across the forehead, between the orbits, is called the _interorbital_ space; that below the orbit, the _infraorbital_ or _sub-orbital_ region. [Sidenote: Gill-cover.] In the _post-orbital_ part of the head there are distinguished, at least in most Teleosteous Fishes and many Ganoids, (Fig. 24) the _præoperculum_, a sub-semicircular bone, generally with a free and often serrated or variously-armed margin; the _operculum_, forming the posterior margin of the gill-opening, and the _sub-operculum_ and _interoperculum_ along its inferior margin. All these bones, collectively called _opercles_, form the _gill-cover_, a thin bony lamella covering the cavity containing the gills. Sometimes they are covered with so thin a membrane that the single bones may be readily distinguished; sometimes they are hidden under a thick integument. In some cases the interoperculum is rudimentary or entirely absent (Siluroids). [Sidenote: Gill-opening.] The _Gill-opening_ is a foramen, or a slit behind or below the head, by which the water which has been taken up through the mouth for the purpose of breathing is again expelled. This slit may extend from the upper end of the operculum all round the side of the head to the symphysis of the lower jaw; or it may be shortened and finally reduced to a small opening on any part of the margin of the gill-cover. Sometimes (_Symbranchus_) the two openings, thus reduced, coalesce, and form what externally appears as a single opening only. The margin of the gill-cover is provided with a cutaneous fringe, in order to more effectually close the gill-opening; and this fringe is supported by one or several or many bony rays, the _branchiostegals_. The space on the chest between the two rami of the lower jaw and between the gill-openings is called the _isthmus_. [Illustration: Fig. 2.--Head of _Mordacia mordax_, showing the single nostril, and seven branchial openings.] The Sharks and Rays differ from the Teleosteous and Ganoid fishes in having five branchial slits (six or seven in _Hexanchus_ and _Heptanchus_), which are lateral in the Sharks, and at the lower surface of the head in the Rays (Fig. 1, p. 34). In Myxine only the gill-opening is at a great distance from the head; it is either single in this family (Cyclostomi), or there are six and more on each side (Fig. 2). [Sidenote: Tail.]