108 



VERTEBRATE ZOOLOGY 



ectodermal epithelium of the branchial clefts. They are found in the 

 embryos of many elasmobranchs (Fig. 57) and in some teleosts. A 

 notable case of larval gills is seen in the advanced larva of Polyp- 

 terus (Fig. 70, C). 



Internal gills are true functional gills of adult fishes. They are finely 

 divided diverticula of the endodermal epithelium of the branchial 

 clefts. Their location is well shown in the diagrams of elasmobranch 



and teleost heads (Fig. 58). 



THE AIR-BLADDER AND ACCES- 

 SORY ORGANS OF RESPIRATION 



In all of the groups of fishes 

 above the elasmobranchs there is 

 a single or paired air-bladder, a 

 sac-like diverticulum of the phar- 

 ynx derived from either dorsal or 

 ventral sides of the alimentary 

 tract. It is in all cases supplied 

 with blood from the "pulmonary 

 artery" and, primitively at least, 

 subserves two functions: that of 

 a hydrostatic or buoyancy organ 

 and that of an accessory respira- 

 tory organ or primitive lung. In 



FIG. 59. -Respiratory labyrinth of 

 the Climbing Perch (Anabas scandens) 

 exposed by removal of part of operculum. 

 ba ', first branchial arch; to, labyrinth- 

 iform organ; op, operculum. sbc, supra- 

 branchial cavity. (From Bridge). 



FIG. 60. Accessory respiratory organs of the cat-fish, Clarias, as seen after 

 removal of operculum. a, anterior arborescent organ; b. a 1-4 , first four branchial 

 arches; d. b. c, dorsal extension of left branchial cavity;/, modified gill-filaments; 

 op, base of operculum; p, posterior arborescent organ. (From Bridge.) 



