120 



Basic Structure of Vertebrates 



ryngobranchial, epibranchial, ceratobranchial, and hypobran- 

 chial, and the median basibranchial. In the hyoid and branchial 

 arches there is a tendency toward reduction in the number of parts 



and fusion of parts, especially in 

 the posterior region of the series. 

 There may be much fusion in the 

 longitudinal series of basibranchials. 

 The cranium is a quite immo- 

 bile structure. Therefore the wall 

 of a cartilaginous cranium may be 

 continuous throughout. The walls 

 of the mouth and pharynx must 

 be capable of executing the move- 

 ments required for swallowing and, 

 in gill-breathers, for maintaining 

 the respiratory flow of water 

 through the branchial chambers. 

 Therefore the visceral arches must 

 consist of parts free to move in 

 relation to one another. The some- 

 what elaborate set of branchio- 

 meric muscles extending in vari- 

 ous ways between parts of the 

 visceral skeleton provides for the 

 necessary movements. 



Fig. 118. Branchial arches of a 

 shark, Heptanchus. (c) Ceratobran- 

 chial; (cbr) cardiobranchial (posterior 

 basibranchial); (e) epibranchial; (h) 

 hyoid; (hb) hypobranchial; (he) hyoid 

 copula; (p) pharyngobranchial; (1-7) 

 branchial arches. (After Gegenbaur. 

 Courtesy, Kingsley: "Comparative 

 Anatomy of Vertebrates," Phila- 

 delphia, The Blakiston Company.) 



BONY SKULL 



The foregoing account of the skull has concerned itself mainly with 

 the cartilaginous skull as it appears in vertebrate embryos and in 

 adult sharklike fishes. In the great majority of vertebrates, the adult 

 skull is more or less completely bony. Transformation of the carti- 

 laginous skull into a bony skull is effected by two quite distinct but 

 closely coordinated processes: ossification of the cartilage to form 

 cartilage-bones or "replacing bones," and development of dermal 

 bones which are always external or superficial in relation to the carti- 

 laginous elements — therefore "investing bones." In "ganoid" fishes 

 (e.g., sturgeon), the dermal scales of the head fuse to form large bony 

 plates whose arrangement is closely similar to that of the dermal 

 bones of the skulls of tetrapod vertebrates. Presumably, therefore, 

 the dermal bones of the skull have been derived from bony scales of 

 primitive fishes (Fig. 119). 



The bony skull is an exceedingly complex structure and presents 

 a bewildering array of adaptive modifications in the various groups 



