THE SKELETAL SYSTEM 



171 



In elasmobranchs the number of visceral cartilages is reduced to corre- 

 spond with the reduced number of gill apertures. The maximum number 

 is nine, in Heptanchus, of which the first, the mandibular, is modified to 

 become the cartilages of the upper and lower jaws, while the second arch, 

 the hyoid, functions slightly as a gill arch, and its dorsal division, the 

 hyomandibular cartilage, acts as a suspensory apparatus for the lower jaw. 



The dorsal division of the first visceral arch, which forms the cartilage 

 of the upper jaw, is called the palato-pterygo-quadrate cartilage, since 

 palatal, pterygoid, and quadrate bones develop from it in amphibians and 

 reptiles. The lower half of the first visceral arch forms cartilage of the 

 lower jaw, Meckel's cartilage. An articulation is formed between the two, 



Fig. 161. — Branchial arches of (A) Heptanchus; (B) Chlamydoselachus; and (C) 

 Cestracion. c, ceratobranchial; e, epibranchial; h, hyoid; hb, hyobranchial; he, hyoid 

 copula; cbr, cardiobranchial (posterior copula); p, pharyngobranchial; 1-7, branchial 

 arches; m, Meckel's cartilage. (A and C after Gegenbaur, B after Garman; from 

 Kingsley's "Comparative Anatomy of Vertebrates.") 



so that in elasmobranchs a biting mouth replaces the sucking mouth of 

 cyclostomes. Nevertheless, the upper jaw does not fuse with the cranium 

 in elasmobranchs but remains independent. 



Posterior to the hyoid arch, the visceral cartilages persist in elas- 

 mobranchs as supports of the branchial or gill arches. The number is 

 commonly reduced to five. Most teleosts have only four functional 

 branchial arches, and in some the number is reduced to two. Perenni- 

 branch amphibians have either two or three. Land animals have lost their 

 gills entirely, but cartilaginous and bony skeletal supports persist and are 

 turned to new and diverse uses. 



In Amphibia, both upper and lower jaw cartilages are at least in part 

 converted into bone, and the upper jaw becomes firmly fastened to the 

 cranium. The hyomandibular in this group ceasing to function as a 

 suspensory apparatus of the jaw, slips into the tympanic cavity to form a 



