The _visceral arches_ consist of a number of pairs of curved rods of cartilage, at first simple, but subsequently segmented, and developed in the splanchnic mesoblastic walls of the oral cavity and pharynx. Each rod is connected with its fellow by a median cartilage in the floor of the pharynx, so that the whole form a series of dorsally incomplete hoops encircling the anterior portion of the alimentary canal. No doubt all the visceral arches were originally branchial arches, and were so disposed between the successive gill-clefts as to support their walls and the vascular folds or gill-lamellae to which they gave rise. In Fishes most of the arches still retain their primitive gill-supporting function, but the first or mandibular arch has become modified to form upper and lower jaws, although in the Sharks and Dog-Fishes it may lie in front of a gill-cleft and still be associated with vestigial gills. The second or hyoid arch is less removed from the condition of a branchial arch, and generally supports either a functional or a vestigial gill, but in most Fishes it has acquired the secondary function of forming a suspensorium for the attachment of the jaws to the cranium. The skull of the common Dog-Fish, _Scyllium canicula_ (Fig. 120),[188] may be studied as a type which in the adult remains cartilaginous, and has no secondary addition of cartilage- or membrane-bones. In this Fish the chondrocranium, or primary cartilaginous cranium, presents the appearance of a somewhat depressed oblong box, which has a complete roof, side-walls, and floor, but is open in front (_anterior cranial fontanelle_) and also behind (_foramen magnum_). The hinder, or parachordal portion of the cranium surrounds the foramen magnum, and there forms the _occipital region_. At the ventral margin of the foramen there are two prominences, or _occipital condyles_, for articulation with the first vertebra, and between them the remains of the notochord are traceable into the cranial floor. {208}[Illustration: FIG. 120.—Side view of the skull of the common Dog-Fish (_Scyllium canicula_). _aud.cp_, Auditory capsule; _br.a_ 1, 5, branchial arches; _br.r_, _br.r′_, cartilaginous rays attached to the hyoid arch and the first four branchial arches; _Cr_, cranium; _ex.br_, extra-branchial cartilages; _hy.cn_, cerato-hyal; _hy.m_, hyomandibular; _lb_, labial cartilages; _lg_, ligaments passing from the jaws to the cranium and to the distal end of the hyomandibular; _lg′_, ethmo-palatine ligament; _l.j_, lower jaw or Meckel's cartilage; _Nv_. 2, optic foramen; _Nv_. 5, foramen for the Vth and part of the VIIth cranial nerves; _olf.cp_, olfactory capsule; _or_, orbit; _up.j_, upper jaw or palato-quadrate cartilage. (From Wiedersheim, after W. K. Parker.)]