The position of the branchial bars in relation to the somatopleure and splanchnopleure can be determined from their relation to the so-called head cavities. These cavities atrophy before the formation of the cartilaginous branchial bars, but it will be observed ([fig. 328]), that the artery of each arch (aa) is placed on the inner side of the head cavity (pp). The cartilaginous bar arises at a later period on the inner side of the artery, and therefore on the inner side of the section of the body cavity primitively present in the arches.
An anterior arch, known as the mandibular arch, placed in front of the hyomandibular cleft, and a second arch, known as the hyoid arch, placed in front of the hyobranchial cleft, are developed in all types. The succeeding arches are known as the true branchial arches, and are only fully developed in the Ichthyopsida.
In some Sharks (Notidani) seven branchial arches may be present (not including the hyoid and mandibular). In other Ichthyopsida five are usually present, in the embryo at any rate, while in the Amniota there are usually two or three post-hyoid membranous arches, in the interior of which a cartilaginous bar is usually formed. The general form of these bars at an early stage of development is shewn in the dogfish (Scyllium) in [fig. 329].
Fig. 328. Horizontal section through the penultimate visceral arch of an embryo of Pristiurus.
ep. epiblast; vc. pouch of hypoblast which will form the walls of a visceral cleft; pp. segment of body-cavity in visceral arch; aa. aortic arch.
The simple condition of these bars in the embryo renders it highly probable that forms existed at one time with a simple branchial skeleton of this kind: at the present day however such forms no longer exist. The first arch has in all cases changed its function and has become converted into a supporting skeleton for the mouth; the hyoid arch, though retaining in some forms its branchial function, has in most acquired additional functions and has undergone in consequence various peculiar modifications. The true branchial arches retain their branchial functions in Pisces and some Amphibia, but are secondarily modified and largely aborted in the abranchiate forms. Since the changes undergone by the true branchial bars are far less complicated than those of the hyoid and mandibular bars it will be convenient to treat of them in the first instance.
Fig. 329. Head of embryo Dogfish, 11 lines long. (From Parker.)
Tr. trabecula; Pl.Pt. pterygo-quadrate; M.Pt. metapterygoid region; Mn. mandibular cartilage; Hy. hyoid arch; Br. 1. first branchial arch; Sp. mandibulohyoid cleft; Cl1. hyobranchial cleft; Lch. groove below the eye; Na. olfactory rudiment; E. eyeball; Au. auditory mass; C 1, 2, 3. cerebral vesicles; Hm. hemispheres; f.n.p. nasofrontal process.