BEANCHIJ3 OF VERTEBRATA. 



543 



poucli iu fi'ont, and the anterior wall of the one behind. While the 

 pouches communicate with the pharyngeal cavity by narrow orifices, 

 which are bounded by the cartilaginous branchial arches, they also 

 open on the side (or in Rays, on the ventral surface) of the body by 

 just as many clefts. Rows of branchial lamel-lge lie in the walls of 

 the branchial pouches ; in the embryonic condition these lamelhe 

 develop filamentous prolongations, which form the external gills. 

 These are also found on the spiracular cleft. The anterior wall 

 only of the last branchial pouch is provided with a gill (Fig. 306, A). 



From this arrangement we may derive what we find in the 

 Ganoidei, and from that what we find in the Teleostei. The spiracular 

 gill, which has no respiratory function iu the adult state of the 

 Selachii, is the first 

 to undergo the A 



greatest degenera- 

 tions. In some 

 Ganoidei, which 

 possess a spiracle 

 (e.g. Acipenser), 

 the gill is con- 

 verted into a 

 Pseudobranchia 

 (a gill which has 

 lost its proper 

 branchial artery 

 and vein) ; this is 

 not found in Poly- 

 pterus nor Amia. 

 In the Osseous 

 Fishes it also ap- 

 pears tobe wanting, 

 or has lost all re- 

 semblance to a gill. 



The anterior 



series of branchial lamellae of the Selachii, that, namely, which is 

 attached to the posterior face of the hyoid arch, is also found among 

 the Ganoidei, where it forms a respiratory opercular gill (Acipenser, 

 Lepidosteus). It is found also during the embryonic stages of the 

 Teleostei, but it does not persist. It sometimes consists of a short 

 row of lamellge, which is attached to the operculum ; sometimes it 

 is carried to the base of the skull, and sometimes it is hidden 

 below the mucous membrane. Even when in this condition, rudi- 

 mentary cartilaginous rods may be found in it. When still more 

 degenerated, it forms a glandular structure, which is made up of 

 several lobules (Esox). 



When all trace of the external branchial skeleton disappears, 

 the septum which arises from each of the inner branchial arches 

 disappears also, or is reduced to a slender fringe. Owing to this, 

 the rows of branchial lamellte in the Ganoidei and Teleostei come 

 into close relation with their respective branchial arches, and are 



Fig. 306. Horizontal section through the branchial cavity. 



A Of Scyllium. £ Of Barbus. The floor of this cavity 



is shown. I Tongue, oe CBsophagus. s Septa of the 



branchial pouches, h Gills, op Operculum. 



