xiii PHYLUM CHORDATA 71 



the segments of the visceral skeleton and the metameres. The 

 visceral arches vary in number from four to nine : the foremost 

 of them is distinguished as the mandibular arch and lies just 

 behind the mouth ; the second is called the hyoid arch, and the 

 rest branchial arches, from the fact that they support the gills in 

 water-breathing forms. 



In all Craniata except the Cyclostomes the mandibular arch 

 becomes modified into structures called jaws for the support of the 

 mouth. Each mandibular bar divides into a dorsal and a ventral 

 portion, called respectively the palato-quadraie cartilage (Fig. 719, 

 A, pal. qu.) and MeckeVs cartilage (mck. c.) : the palato-quadrates 

 grow forwards along the upper or anterior margin of the mouth, 

 and unite with one another in the middle line, forming an upper 

 jaw: Meckel's cartilages similarly extend along the lower or 

 posterior margin of the mouth and unite in the middle line, 

 forming the lower jaw. The quadrate (qu) or posterior end of 

 the palato-quadrate furnishes an articulation for the lower jaw, 

 and often acquires a connection with the cranium, thus serving 

 to suspend the jaws from the latter. Thus each jaw arises from 

 the union of paired bars, the final result being two unpaired 

 transverse structures, one lying in the anterior, the other in the 

 posterior margin of the transversely elongated mouth, and moving in 

 a vertical plane. The fundamental difference between the jaws 

 of a Vertebrate and the structures called by the same name in an 

 Arthropod or a Polychsetous Worm will be obvious at once. 



The hyoid bar usually becomes divided into two parts, a dorsal, 

 the hyomandibular OT pharyngo-hyal (hy.m.), and a ventral, the hyoid 

 cornu, which is again divisible from above downwards into segments 

 called respectively epi-hyal (ep.hy\ cerato-hyal (c.hy), and hypo-hyal 

 (hJiy). The median ventral element of the arch, or basi-hyal (b.hy), 

 serves for the support of the tongue. In some Fishes the hyoman- 

 dibular articulates above with the auditory region of the cranium, 

 while the jaws are connected with its ventral end. We may thus 

 distinguish two kinds of suspensorium or jaw-suspending appara- 

 tus, a mandibular suspensorium, furnished by the quadrate, and a 

 hyoidean suspensorium by the hyomandibular : in the former case 

 the skull is said to be autostylic, i.e. having the jaw connected by 

 means of its own arch, in the latter it is called hyostylic : in a few 

 instances an amphistylic arrangement is produced by the articula- 

 tion of both mandibular and hvoid arches with the skull. 



t/ 



The branchial arches become divided transversely into dorso- 



i/ 



ventral segments called respectively pharyngo-branchial (ph. br.), 

 epi-ljranchial (ep.ltr.), cerato-branchial (c.br.), and hypo-branchial 

 h.br.), and the visceral skeleton thus acquires the character of 

 an articulated framework which allows of the dilatation of the 

 pharynx during swallowing and of its more or less complete 

 closure at other times. 



