144 STRUCTURE OF THE VERTEBRATES 



and tongue, the posterior five remaining as the branchial, or gill, 

 arches. 



The evidence for the homology between the jaws and the 

 visceral skeleton is based upon several factors: (1) in the very 

 early embryo of the dogfish the first visceral cartilage is clearly 

 divided into two major portions, but these form a wide angle; 

 (2) the second (hyoid) arch lies anterior to the spiracle which 

 carries rudimentary gills; (3) the muscles of the jaw and hyoid 

 arise with the branchial muscles from the hypomere as the 

 visceral muscles; and (4) the innervation of these muscles 

 completes the proof of homology. The muscles and nerves are 



TT^. 





^:£^j^F^^ 



Meckel's cartilage « 



'^n 'm '^ IV " V VI VII 



Fig. 81. Chondrocranium and Visceral Arches (Diagrammatic). Arches I 



and II form the jaws and hyoid apparatus; Arches III to VII form the 



branchial cartilages. 



undoubtedly the strongest evidence. The illustration of the chon- 

 drocranium and visceral arches of the dogfish shows the rela- 

 tionships of these cartilages to the chondrocranium. 



The enclosure of the upper and lower jaws by dermal bone 

 has been discussed. AVith this development of the dermal skull 

 the hyomandibular of the fish became a practically functionless 

 structure, lying in close contact with the spiracle. This is the 

 condition in the Crossopterygian fishes, the hyomandibular 

 bone being very small. 



The amphibia evolved an entirely new relationship between 

 these structures. The outer (distal) end of the spiracle enlarges 

 and engulfs the hyomandibular into its cavity, and a membrane 

 o-f skin closes the outer opening of the cavity. In this way the 

 middle car of the amphibian is developed. The skin membrane 



