RESPIRATORY SYSTEM).
Among the Cyclostomata (hags and lampreys) there is an arrangement known as the "branchial basket," which has a more superficial position than the visceral arches of fish and probably corresponds to the extra-branchials of those vertebrates. The oral and hyoid arches are very rudimentary and probably have de generated in consequence of the suctorial mode of nourishment. In the Elasmobranchii (sharks and rays) the visceral skeleton is entirely cartilaginous. In the more primitive types such as the comb-toothed shark (Notidanus) the oral and hyoid arches are quite distinct. The oral arch consists of the upper jaw, or palato quadrate cartilage, and the lower jaw, or Meckel's cartilage; these articulate with one another posteriorly and also with the skull. Behind these and distinct from them is the hyoid arch. Such a type of suspensorium or jaw articulation is called autostylic. In the rays, on the other hand, the oral arch is connected with the skull by the proximal segment of the hyoid arch, which, since it connects both the hyoid and mandibular (oral) arches with the skull, is called the hyoman dibular cartilage. This type of suspensorium is termed hyo stylic.
Below the hyomandibular car tilage the hyoid arch has two other segments, the ceratohyal laterally and the basihyal ven trally where it fuses with its fellow of the opposite side. Some times an epihyal intervenes between the hyomandibular and the ceratohyal. Behind the hyoid arch are usually five branchial arches, though in Heptanchus there are as many as seven. These are divided into a number of segments and outside these there is often another series of arches called extra-branchials which are probably homologous with the branchial basket of the Cyclosto mata.
The chimaeroid fishes are called Holocephali because in them the palato-quadrate bar is fused with the rest of the skull. In the
bony ganoids and teleosteans (Teleostomi) the palato-quadrate bar ossifies to form the palatine, ecto-, meso- and meta-pterygoids and quadrate bones from before backward, while outside these is another row of dermal bones formed by the premaxilla, maxilla and fugal or malar.
In the lower jaw, Meckel's cartilage is ossified at its proximal end to form the articular bone, but distally it remains and is partly encased by the dentary, and more posteriorly by the angu lar, both of which are membrane bones. The jaw joint therefore is between the quadrate and the articular. In comparing this de scription with the section on human embryology it will be seen that certain bones, like the palate and pterygoids, which in the fish are ossifications in cartilage, become in the higher vertebrates membrane bones, and so it is clear that too great stress must not be laid on the histological history of a bone in determining its morphological significance.
The branchial arches of the Teleostomi closely resemble those of the Elasmobranchii except that they are ossified and that the extra-branchials have disappeared.
In the Dipnoi (mudfish) the suspensorium is autostylic, and either five or six branchial arches are present. In the Amphibia, too, the suspensorium is autostylic, the palato-quadrate bar re mains largely cartilaginous, though its posterior part is often ossified to form the quadrate. The membranous presnaxilla, max illa, palatine, pterygoid, quadratojugal and squamosal bones are developed in connection with it, though it is interesting to notice that the pterygoid is sometimes partly cartilaginous and the quadrato-jugal is absent in the tailed forms (Urodela). In the lower jaw a splenial element has appeared, and in the frog a cartilaginous mento-meckellian bone develops close to the sym physis. In the larval stages there are rudiments of four branchial arches behind the hyoid, but in the adult these are reduced in the Anura and their ventral ends are united into a broad basilingual plate.