The principal respiratory organs consist of a series of pairs of branchial clefts in the form of perforations in the side walls of the throat, which place the pharynx in free communication with the exterior. The first and most anterior of these clefts, the mandibulo-hyoid cleft or "spiracle," is situated between the mandibular and hyoid arches; the second, the hyo-branchial or hyoidean cleft, between the hyoid arch and the first branchial arch; and the remaining clefts between the succeeding branchial arches. On the anterior and posterior walls of more or fewer of the clefts highly vascular plate-like, or variously shaped filamentous outgrowths of their lining membrane are developed, which subserve the purpose of exposing the blood to the influence of the oxygen-containing water, and are termed branchial lamellae or "gills." In addition to their usual respiratory organs, the gills, a few Fishes utilise the air-bladder either as a functional lung or as an oxygen reservoir, and in others accessory breathing organs of various kinds are developed. The arrangement of the branchial clefts and the gills may be conveniently studied first in the Elasmobranchs. Excluding the spiracles, there are usually in this group (Fig. 161, A), five pairs of branchial clefts, but in certain primitive members of the group the number may be larger. Thus, in _Notidanus griseus_ (_Hexanchus_) and in _Chlamydoselachus_ there are six, and in _Notidanus cinereus_ (_Heptanchus_), seven clefts. The pharyngeal apertures of the clefts are relatively wide, but their external openings, which are freely exposed on the lateral surface of the head between the eye and the pectoral fin, are usually narrow and slit-like. {278}[Illustration: FIG. 161.—A, Horizontal section through the head of an Elasmobranch; B, similar section of a Teleost (diagrammatic). _b.c_, Branchial cavity; _b.l_, branchial lamellae; _c_, coelom; _e.b.a_, external branchial aperture; _hy.a_, hyoid arch; _hy.c_, hyo-branchial cleft; _l.s_, interbranchial septum; _n_, nasal organ; _oes_, oesophagus; _op_, operculum; _p.q_, palato-quadrate cartilage; _Ph_, pharynx; _sp_, spiracle; _s.ps_, spiracular pseudobranch; 1-5, 1st to 5th branchial arches. (From Boas, slightly altered.)]