230 



COMPARATWE ANATOMY 



in elasmobranchs but remains independent. The suggestion that the 

 labial cartilages of elasmobranchs are rudiments of cartilages which sup- 

 ported pre-oral gill arches in the ancestors of elasmobranchs has too slender 

 a foundation to be taken seriously. This, however, is not to deny the 

 possibility that there were preoral segments in such ancestors. 



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, but whether the loss takes place at the end or 

 in the middle of the series is a disputed point. Most teleosts have only 

 four functional branchial arches, and in some the number is reduced to 

 two. Perennibranch amphibia have either two or three. Land animals 



Fig. 187. — 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.") 



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 

 connexion between the ear-drum and the inner sensory ear, the columella 

 or stapes. The hyoid cartilage forms the basis of attachment of the 

 tongue muscles. With the disappearance of the gills as functional organs 

 in land amphibia, the remaining visceral cartilages become further 

 reduced and associated with the larynx. 



Some advance towards the mammalian visceral skeleton appears in 

 hving reptiles. The teeth both in upper and lower jaws become lodged 

 in alveolar sockets in two membrane bones, the premaxillary and maxillary 



