470 



COMPAEATIVE ANATOMY. 



tlie 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-lamellte on one side only ; in 



Fig. 256. Hyoid and branchial arches of Perca fhiviatilis. I — FT Arches; the 

 first (I) is converted into an organ for the support of the hyoid ; the next four (71 — V) 

 are branchial arches, and the last {VI) forms the infra-pharyngeal bone, a h c d 

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

 r Branchiostegal rays, f g 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 Clupeidee ; 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 Ganoidei and Teleostei, 

 where they form fine cartilaginous lamellge, placed between the I'ows 

 of the branchial folds. 



§ 355. 



The branchial skeleton of the Amjjhibia is considerably reduced) 

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



