{205}In the dominant group of existing Fishes, the Teleostei, the centra are almost invariably biconcave, although in the Eels they may be flat or even slightly convex in front. Ribs are absent in the Syngnathidae and in the Plectognathi. In addition to the usual articulation between the centra, the vertebrae often articulate with one another by means of paired processes arising from the anterior margin of each neural arch, or from the centrum at the base of the arch (pre-zygapophyses), and meeting similar processes which project either from the hinder margin of the arch of the vertebra in front, or from the adjacent portion of its centrum (post-zygapophyses). The haemal arches may have similar processes (Fig. 119). One, two, or in some Teleosts, three pairs of slender intermuscular bones radiate outwards from the centra into the myocommata (epicentrals), or from the neural arch (epineurals), or from the ribs (epipleurals). [Illustration: FIG. 119.—A, side view of precaudal vertebrae of a Cod (_Gadus morrhua_) without the ribs; B, similar view of caudal vertebrae of the same Teleost. _c_, Centrum; _h.a_, haemal arch; _h.sp_, haemal spine; _n.a_, neural arch; _n.sp_, neural spine; _p_, parapophysis; _p.z_, pre-zygapophysis; _pt.z_, post-zygapophysis.] THE RIBS.—It is doubtful if the structures termed "ribs" are homologous in the different groups of Fishes. There appear to be two kinds, distinguishable as dorsal and ventral ribs (Fig. 156). Dorsal ribs are situated in the fibrous tissue separating the epiaxial from the hypaxial muscles of the body wall, and they take no part in forming the haemal arches of the caudal region. Ventral ribs, on the other hand, always lie internal to the hypaxial muscles, and directly external to the peritoneal lining of the coelom, and they usually contribute to the formation of {206}the haemal arches. To the former belong the ribs of the Elasmobranchs, and to the latter the ribs of the Teleostomi and Dipnoi. _Polypterus_ alone has both kinds of ribs. THE SKULL. The skull is a highly complex structure, the various components of which are as different physiologically as they are morphologically. It consists (i.) of the _cranium_, for the enclosure and protection of the brain; (ii.) of _sense capsules_, which fulfil a like function for the auditory, visual, and olfactory organs; (iii.) of certain _vertebrae_ or vertebral elements fused with the hinder part of the cranium; (iv.) of a series of _visceral arches_; and (v.) of a series of paired or median cartilages developed in relation with the mouth and nostrils, which may be collectively spoken of as "_labial_" cartilages.