284 FISHES CHAP. 



and a few Teleosts 1 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 Elasmobrauchs, 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, 2 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, 3 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. 4 In most other 

 Teleostomi a degenerate hemibranch occupies a similar position. 

 In Amia 5 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 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 (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 inirabile " 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. 



4 F. W. Miiller, Arch. Mikrosk. Anat. xlix. 1897, p. 463. 



5 Ramsay Wright, op. cit. p. 492. 



