SKULL 



81 



as gill-bearers in the Anamnia, but even the first of them, the 

 hyoid, along which the facial nerve extends, becomes modified from 

 those lying behind it : these, the branchial arches proper, of which 

 there are usually five in Fishes, are in relation with the glosso- 

 pharyngeal and vagus. All the visceral arches must originally, 

 however, have borne gills. 



Primarily unsegmented, the individual post- oral arches may 

 become divided up into portions (phoryngo-, epi-, cerato-, and hypo- 

 hyal or branchial), the uppermost of which lies under the base of 

 the skull, while the lowermost is connected with its fellow by a 

 median basal piece or copula (basihyal, basibranchial). 



The mandibular arch also undergoes segmentation, and becomes 

 divided into a short proximal piece, the quadrate, and a long distal 



V 



FIG. 63. DIAGRAMMATIC FIGURE OF AN EMBRYONIC ELASMOBRANCH SKULL, 

 SHOWING THE RELATIONS OF THE VlSCERAL ARCHES. 



A, eye ; a to e, branchial arches, between which the gill clefts (7 to V) are seen ; 

 br, brain ; C, inter-vertebral remains of notochord ; H, hyomandibular ; K, 

 hyoid arch ; L, labial cartilages ; M, Meckel's cartilage ; N, nasal capsule ; 

 0, auditory capsule ; PQ, Q, palatoquadrate, connected with the trabecula 

 by ligaments at f ; S, spiracle ; sp.c, spinal cord ; Tr, trabecula ; V, vertebral 

 arches. 



mandibular or Meckel's cartilage (Fig. 63). The quadrate gives 

 rise anteriorly to a process, the palatoquadrate or palatopterygoid, 

 which usually becomes fixed in various way to the base of the 

 skull and gives rise to the primary upper jaw, Meckel's cartilage 

 forming the lower jaw. 



The quadrate, which primarily serves as a support (suspcn- 

 sorium) for the jaws, either remains separated from the skull, being 

 only connected with it by an articulation or by connective tissue, 

 or becomes united with it. 



The hyoid may also take part in the suspensorial apparatus, 

 and thus come into close relation to the mandibular arch and 



G 



