[Illustration: 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_, {284}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 {285}all cases quite clear. In _Salmo_ it is said that there is