470 



COMPARATIVE ANATOMY. 



the hinder ones (IV V) are united to a single piece (a), and are always 

 degenerated, both in size and number. The last pair of all (VI), which 

 merely consists of a single piece on either side, carries no gills; in 

 the fifth arch also there are often gill-lamellas on one side only ; in 



Fig. 256. Hyoicl and branchial arches of Perca flnviatilis. I — VI Arches; the 

 first (I) is converted into an organ for the support of the hyoid; the next four (II — V) 

 are branchial arches, and the last (VI) forms the infra-pharyngeal bone, abed 

 Segments of the arches. The uppermost piece (d) forms the supra-pharyngeal bones. 

 r Branchiostegal rays, f y h Copula? (after Cuvier). 



the last, however, dental structures are more completely developed, so 

 that this piece is often capable of functioning as a masticatory 

 organ. In the Pharyngognathi the rudiments of the last arch, on 

 either side, are fused into one piece. 



We meet with other modifications of the posterior branchial 

 arches in the Labyrinthobranchiata, and in various Clupeidse ; these 

 are due to the conversion of various segments of the arches into the 

 walls of spaces into which water is received. 



Just as the hyoid arch of the Selachii is provided with cartila- 

 ginous appendages, so also the succeeding arches are beset with 

 cartilaginous rays which support the walls of the branchial pouch. 

 Even these structures are rudimentary in the Ganoi'dei and Teleostei, 

 where they form fine cartilaginous lamellae, placed between the rows 

 of the branchial folds. 



§ 355. 



The branchial skeleton of the Amphibia is considerably reduced ; 

 such forms as undergo a metamorphosis have the gills reduced, and 



