RESPIRATORY ORGANS 



283 



biserial gill or holobranch. In shape the branchial filaments 

 are usually soi^iewhat triangular, and consist of an axial support- 

 ing cartilage o-r 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. Xot 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 



Fig. 164. — Trausversc sections of braucbial arclies in different Fishes. A, Elasmobraiicli ; 

 B, Cliimaera; C, Adpensei- ; D and E, Teleosts. b.a, Branchial arch ; g.l, gill- 

 lamellae ; fjr, 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 " {Hipixjcamjms). 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 Crossopterygii {Polypterus), and in 

 the Chondrostei {Aci-penser and Polyoclon). They have been 

 observed, however, in the embryos of some Teleosts, as in the 

 Salmon (Salmo),^ and even in the adults of Amia,^ Lejndosteus, 



Amphipnous cuchia, where only the secoud arch has a biserial gill, the remaining 

 arches being wholly devoid of gills (c/. p. 598). 



1 Balfour, Comp. Embnjol. ii. 1881, p. 62. 



^ Ramsay Wright, Journ. AnUt. and Phys. xi.x. 1883, p. 476. 



