GILLS 



357 



As a rule Teleosts possess only four holobranchs, 1 and this holds 

 good for all Ganoids. A vestigial gill or pseudobranch is present 

 on the anterior wall of the spiracle of many Elasmobranchs and 

 of cartilaginous Ganoids (inandibular pscudobranch) ; the posterior 

 hyoid hemibranch, which is functional in Acipenser and Lepi- 

 dosteus, becomes more or less reduced in other Ganoids and 

 Teleosts, forming the so-called opereular pseudobranch. Traces of 



FIG. 263. DIAGRAM ILLUSTRATING THE MECHANISM OF RESPIRATION IN 

 TELEOSTS. (After Dahlgren.) 



A, phase of inspiration ; B, phase of expiration. In both figures the anterior 

 oral part (cav.oris) represents a vertical section, and the posterior pharyu- 

 geal part enclosing the gills (Kiemen) a horizontal section. The arrows 

 indicate the direction of the water-current and pressure, and those passing 

 through the walls of the oral cavity the expansion and contraction of the 

 opercular apparatus. In ^4, the maxillary and mandibular valves are open, 

 and the branchiostegal membrane closed : in B, this condition is reversed. 



a cleft, lying behind the functional branchial clefts, are found 

 in the embryos of certain Fishes and Amphibians. All these facts 

 indicate the presence of a more extensive branchial apparatus in 

 ancestral forms. (For the sieve-like gill-rakers, cf.Figs. 261 and 262.) 

 In the Lophobranchii the gills are replaced by tufted processes, 

 and in many Teleostei certain accessory structures are developed 

 in the posterior region of the branchial chamber by a modification 



1 They may be reduced to three, or two, and even these may be more or less 

 rudimentary. 



A A 



** 



