x RESPIRATORY ORGANS 283 
biserial gill or holobranch. In shape the branchial filaments 
are usually somewhat triangular, and consist of an axial support- 
ing cartilage or bone, invested superficially by a highly vascular 
mucous membrane. As in most of the preceding groups the 
fifth branchial arch is gill-less. All Teleostomi possess a well- 
developed movable operculum, supported by a more or less 
complete series of opercular bones, with or without the addition 
of branchiostegal rays (Fig. 161, B). The size of the external 
branchial aperture varies considerably. Usually the hinder and 
lower margins of the operculum are free, and then the aperture 
is spacious. Not infrequently, however, the more or less exten- 
sive fusion of the ventral and hinder edges of the operculum 
with the body-wall reduces the aperture to a narrow slit, as in 
Zo 
r 
Fic. 164.—Trausverse sections of branchial arches in different Fishes. A, Elasmobranch ; 
B, Chimaera ; C, Acipenser; D and E, Teleosts. 6.a, Branchial arch; g./, gill- 
lamellae ; g7, gill-raker ; i.s, inter-branchial septum. (From Boas.) 
the Eels and some Siluridae, or to a small upwardly directed pore, 
as in the “Sea-Horse” (Hippocampus). In the Symbranchidae 
the branchial apertures close dorsally, but fuse ventrally, leaving 
a single median orifice on the under side of the throat. 
Open spiracles are wanting in most adult Teleostomi, but are, 
nevertheless, retained in the Crossopterygit (Polypterus), and in 
the Chondrostei (Acipenser and Polyodon). They have been 
observed, however, in the embryos of some Teleosts, as in the 
Salmon (Sa/mo),! and even in the adults of Amia,’ Lepidosteus, 
Amphipnous cuchia, where only the second arch has a biserial gill, the remaining 
arches being wholly devoid of gills (cf. p. 598). 
1 Balfour, Comp. Embryol. ii. 1881, p. 62. 
* Ramsay Wright, Journ, Anat. and Phys. xix. 1885, p. 476. 
