Fig. 164.—Transverse sections of branchial arches in different Fishes. A, Elasmobranch; B, Chimaera; C, Acipenser; D and E, Teleosts. b.a, Branchial arch; g.l, gill-lamellae; gr, gill-raker; i.s, inter-branchial septum. (From Boas.)
Open spiracles are wanting in most adult Teleostomi, but are, nevertheless, retained in the Crossopterygii (Polypterus), and in the Chondrostei (Acipenser and Polyodon). They have been observed, however, in the embryos of some Teleosts, as in the Salmon (Salmo),[[286]] and even in the adults of Amia,[[287]] Lepidosteus, and a few Teleosts[[288]] are represented by pouch-like recesses of the oral cavity. A few vestigial branchial lamellae may be developed on the anterior wall of each spiracle in Acipenser and Polyodon, but are wanting in Polypterus, and, as in Elasmobranchs, represent a mandibular or spiracular pseudobranch.
The structure usually regarded as a hyoidean hemibranch in the Teleostomi differs greatly in its development in different members of the group. In Acipenser it is undoubtedly the hemibranch of the hyoid arch and is a true gill, receiving venous blood from the ventral aorta and returning arterial blood to the dorsal aorta, as in Elasmobranchs. In Polyodon and in Polypterus the hemibranch is suppressed. Lepidosteus,[[289]] on the other hand, has two series of lamellae on the inner surface of the operculum, a dorsal and a ventral series meeting at an angle (Fig. 197). The ventral lamellae are supplied with venous blood, the dorsal with arterial,[[290]] so that while the former retain their primitive character as a functional hyoidean hemibranch, the latter is a pseudobranch. It is interesting to note, however, that the development of this pseudobranch and its blood-vessels proves that it does not represent any portion of a true hyoidean hemibranch, but is really a spiracular pseudobranch.[[291]] In most other Teleostomi a degenerate hemibranch occupies a similar position. In Amia[[292]] it is very feebly developed, and is lodged in a canal communicating with the branchial cavity by a small aperture, and situated directly anterior to the dorsal end of the first branchial arch. Its blood supply is arterial, and the organ is therefore a pseudobranch. In Teleosts the hemibranch is invariably a pseudobranch; nevertheless, its primitive condition as a gill is indicated either by its structure or by its embryonic history. In some genera the pseudobranch consists of short free lamellae, as in some Pleuronectidae; or it is partly free and partly concealed, as in some of the Horse Mackerels (Caranx) and in Salmo; or it may be completely hidden beneath the oral epithelium, as in the Cod (Gadus), where the organ is very degenerate, and is little more than a "rete mirabile" of blood-vessels. The nature of the Teleostean pseudobranch is not in all cases quite clear. In Salmo it is said that there is no hyoidean hemibranch, and that the pseudobranch is really a persistent spiracular pseudobranch;[[293]] hence it is probable that a like significance must be attached to this singular structure in other Teleosts. The evidence of the cranial nerves on this point is conflicting. If the pseudobranch pertains to the spiracular cleft its nerve supply should be derived from the nerve of that cleft—viz. the seventh or facial nerve; but if it represents a hyoidean hemibranch, then one would expect it to be innervated by the ninth or glossopharyngeal nerve. As a matter of fact, however, the organ is said to be supplied by the seventh in some Teleosts, and in others by the ninth nerve.
In the Dipnoi the branchial system is best developed in Neoceratodus, the increasing importance of the lungs as respiratory organs in Protopterus and Lepidosiren being associated with a corresponding reduction in the structural and functional development of the gills. There is no trace of spiracles in the adult.
Fig. 165.—Transverse section through a branchial arch of Neoceratodus (semi-diagrammatic), a.b.a, Afferent branchial artery; b.a, branchial arch; b.f, branchial filaments; e.b.v, efferent branchial vessel; g.r, gill-rakers. (From Baldwin Spencer.)
In Neoceratodus[[294]] there are five branchial clefts, including the hyobranchial. Each of the first four branchial arches carries a pair of hemibranchs, and, as in the Holocephali, the gill-lamellae are attached along nearly their whole length to a well-developed interbranchial septum (Fig. 165). A peculiarity of Neoceratodus, which has no counterpart in any other Fishes, is the extension of the branchial lamellae on to the dorsal and ventral walls of the branchial clefts, so that the hemibranchs on opposite sides of each cleft are continuous both dorsally and ventrally (Fig. 166). The fifth arch is gill-less. In addition to the normal gills there is also a hyoidean pseudobranch. As in other Dipnoi, an operculum forms the outer wall of the branchial cavity, and leaves but a narrow, slit-like external branchial aperture.
Fig. 166.—The second branchial cleft of Neoceratodus, to show the dorsal and ventral continuity of two hemibranchs on opposite sides of the same cleft. b.c, Branchial cleft; b.f, branchial filaments; g.r, gill-rakers. (From Baldwin Spencer.)