66 



VERTEBRATE SKELETON 



The hyoid arch Hes between the first 

 true gill cleft and the spiracle (or its 



more conservative, the posterior the more variable and the first 

 to disappear. 



In all Gnathostomes branchial and hyoid arches arise deep in the pharyn- 

 geal walls, medial to the coelom and its rudiments (fig. 70). In other words 



they constitute a splanchnic skeleton. As stated 

 above they are usually regarded as belonging 

 to the skull, the position of some behind the 

 limits of the head being explained bj'^ shifting, 

 but possibly their innervation by the vagus 

 nerve is the result of several post-cranial nerves 

 becoming associated with the anterior part of 

 the tenth which certainly is a cranial nerve. 

 Fig. 70. — Section of gill septum 

 of Pristiurus (Dohrn, '84), show- 

 ing relations of branchial cartilage 

 to remnants of coelom. a, bran- 

 chial artery; av, anterior vein .,., tt-i. ^u 



(efferent artery) ; ft. precartilage ^omologuc m higher Vertebrates, the 

 of gill arch; c. remains of coelom. J^^u^tachian or auditorv tubc) . It divides 



the mesothelial tissue continuing ^ -' 



towards medial side of section, w; early into two main parts (fig. 71), a 

 n. nerve; pv, posterior vein. jorsal hyomandlbula, which in most 



Elasmobranchs is attached to the otic capsule by hgaments and in 

 many plays a part in the suspension of the jaws. The lower region, 

 the hyale, has its two halves connected below by a basihyal copula. 

 Sometimes the hyale remains entire, but usually it is divided into 

 three cartilages, the epi-, cerato- and hypohyals. The hyale largely 

 supports the floor of the mouth, and in higher groups, the tongue 

 as well. 



The first (mandibular) arch forms the framework of the mouth in 

 Elasmobranchs (fig. 71), the spiracle lying between it and the 

 hyoid arch. Except in the earHest stages, each half arch is divided 

 into dorsal and ventral parts which meet at a sharp angle at the hinge 

 which Hes behind. The dorsal part, the pterygoquadrate cartilage,^ 

 forms the skeleton of the upper jaw of Elasmobranchs and Chon- 

 drostei. The lower part — Meckel's cartilage or Meckelian— is the 

 framework, at least in the early stages, of the lower jaw of all Gnatho- 

 stomes and the perma.ient jaw of Elasmobranchs and Chondrostei. 

 The ends of the arch, corresponding to the dorsal and ventral ends of 

 a gill arch — the tips of the upper and lower' jaws of a shark — meet 



1 Often called palatoquadrate or palatopterygoid, because once thought to be 

 concerned in the formation of the palatine bone of higher Vertebrates. The name 

 adopted here seems preferable, although it is uncertain to what extent it is homologous 

 with the pterygoid process of man. 



