THE EVIDENCE OF THE SKELETON 



123 



An anterior arch known as the 

 C3 h-v* 



In addition, a branchial skeleton exists, which consists of a series 

 of bars known as the branchial bars, so situated as to afford support 

 to the successive branchial pouches, 

 mandibular arch (Fig. 50, 

 Mn.), placed in front of the 

 hyo-rnandibular cleft, and 

 a second arch, known as the 

 hyoid arch (Hy.), placed in 

 front of the hyo-branchial 

 cleft, are developed in all 

 types; the succeeding arches 

 are known as the true bran- 

 chial arches (Br.), and are 

 only fully developed in the 

 Ichthyopsida. In all cases 

 of jaw-bearing (gnathosto- 

 matous) vertebrates the first 

 arch has become a support- 

 ing skeleton for the mouth (Fig. 51), and in the higher vertebrates in 

 combination with the second or hyoid arch takes part in the formation 

 of the ear-bones. 



The true branchial arches persist, to a certain extent, in the 



cr 



c r ~ 



Mn Hu Br i 

 Hm Na Tr 



Fig. 50. — Head op Embryo Dog-fish, eleven 

 lines long. (From Parker.) 



Tr., trabecula ; Mn., mandibular cartilage ; Hy., 

 hyoid arch; -Br,., first branchial arch; Na., 

 olfactory sac ; E., eye ; An., auditory capsule ; 

 Hm., hemisphere; C,, C 2 , C z , cerebral vesicles. 



Ku'Htj 



Fig. 51. — Skull op Adult Dog-pish, Side View. (From Parker.) 

 cr., cranium; Br., branchial arches; Mn. + Hy., mandibular and hyoid arches. 



Amphibia, and become still more degenerated in the Amniota 

 (reptiles, birds, and mammals) in correlation with the total dis- 

 appearance of a branchial respiration at all periods of their life. 



