284 FISHES CHAP. 
and a few Teleosts! 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, repre- 
sent 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,? 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 supphed with venous blood, the dorsal 
with arterial,® 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 hemi- 
branch, but is really a spiracular pseudobranch.* In most other 
Teleostomi a degenerate hemibranch occupies a similar position. 
In Amia? it is very feebly developed, and is lodged in a eanal 
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 invari- 
ably 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 (Caranz) 
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 
1 Sagemehl, Morph. Jahrb. ix. 1884, p. 213. 
2 Ramsay Wright, op. cit. p. 482. 3 See p. 335. 
41. W. Miller, Arch. Mikrosk. Anat. xlix. 1897, p. 463. 
> Ramsay Wright, op. cit. p. 492. 
