These bars are, as already mentioned, most numerous in certain very primitive forms (seven in Notidanus), while as we ascend the series there is a gradual tendency for the posterior of them to disappear. This tendency is the result of a gradual atrophy of the posterior branchial pouches, which commenced at a stage in the evolution of the Chordata long prior to the appearance of cartilaginous or osseous branchial bars, and reaches its climax in the Amniota.
In a fully developed branchial bar the primitively simple rod of cartilage becomes divided into a series of segments, usually four, articulated so as to be more or less mobile: and either remaining cartilaginous or becoming partially or wholly ossified. Each bar ([fig. 327]) forms a somewhat curved structure, embracing the pharynx. The dorsal and somewhat horizontally placed segment is known as the pharyngobranchial (Ph.Br), the next two as the epibranchial (E.Br) and ceratobranchial (C.Br), and the ventral segment as the hypobranchial (H.Br). There is also typically present a basal unpaired segment, uniting the bars of the two sides, known as the basibranchial (B.Br). The arches often bear cartilaginous rays which support the gill lamellæ.
In Teleostei dental plates are usually developed as an exoskeletal covering on parts of the branchial arches.
In the Amphibia four or three branchial arches are present in the embryo. These parts are more or less completely retained in the Perennibranchiata and Caducibranchiata, but in the Myctodera and Anura they become largely reduced, and entirely connected with the hyoid.
In the Anura they never reach any considerable development, and are soon reduced to a plate ([fig. 330])—the coalesced basihyal and basibranchial plate—the posterior processes of which represent the remnants of the branchial arches.
Fig. 330. Young Frog, with tail just absorbed; side view of skull. (From Parker.)
Au. auditory capsule; in front of it is the cranial side wall; A.N. external nostril; St. stapes; Mck. Meckelian cartilage; B.Hy. basihyobranchial plate; St.Hy. stylohyal or ceratohyal; Br. 1. first branchial arch.
Bones: E.O. exoccipital; Pr.O. prootic; Pa. parietal; Fr. frontal; Na. nasal; Pmx. premaxillary; Mx. maxillary; Pt. pterygoid; Sq. squamosal; Qu.Ju. quadratojugal; Art. articular; D. dentary.
According to Parker the posterior process of this plate in the adult is a remnant of the fourth branchial bar; the next one is the third branchial bar, while the anterior lamina behind the hyoid is stated by him (though this is somewhat doubtful) to be a remnant of the first two bars.
In the Amniota, the branchial arches become still more degenerated, in correlation with the total disappearance of a branchial respiration at all periods of life. Their remnants become more or less important parts of the hyoid bone, and are solely employed in support of the tongue. Their basal portions are best preserved, forming parts of the body of the hyoid. The posterior (thyroid) cornua of the hyoid are remnants of the true arches. Of these there are two in the Chelonia and Lacertilia, and one in the Aves and Mammalia. In Aves the cornu formed from the first branchial arch ([fig. 331], cbr) is always larger than that of the true hyoid arch (ch).