690 



COMPARATIVE ANATOMY 



and pharynx the gill arches are carried back some distance behind the 

 head. All of the arches are cartilaginous at first." 



The mandibular arch lies in the region of the fifth nerve, behind 

 the mouth^nd between it and the first visceral cleft the cleft that be- 

 comes the spiracle or Eustachian tube. "The arch is divided into dorsal 

 and ventral halves, known respectively as the pterygoquadrate (palato- 

 quadrate) and Meckelian cartilages. In the elasmobranch and chondro- 



Fig. 409. 



Side view of a mackerel, ar, articulare ; as, alisphenoid ; bo, 

 basioccipital ; d, dentary ; enp, entopterygoid ; eo, exoccipital ; ep, 

 ectopterygoid ; es, extrascapular ; epo, epiotic ; /, frontal ; io, in- 

 teropercular ; eth, ethmoid ; I, lacrimal ; mx, maxillary ; mxp, 

 metapterygoid ; n, nasal ; op, opercular ; p, parietal ; pe, petrosal ; 

 pf, postfrontal ; pi, palatine ; pm, premaxillary ; po, preoperculum ; 

 poo, postorbital ; prf, prefrontal ; ps, parasphenoid ; q, quadrate ; 

 sbo, suborbital ; so, supraoccipital ; sop, suboperculum ; spo, 

 sphenotic ; sq, squamosal ; sac, suprascapular ; sy, symplectic. 

 (From Kingsley after AUis.) 



steous ganoids the pterygoquadrate forms the upper jaw, lying parallel 

 to and joined to the cranium by ligaments or in chimaeroids by continu- 

 ous growth. With the appearance of bones a new upper jaw is formed, 

 as described below, and the pterygoquadrate becomes more or less re- 

 duced, and ossifies as two or more bones with greatly modified functions. 

 Meckel's cartilage is the lower jaw of the lower vertebrates, while in the 

 higher it forms the axis around which the membrane bones of the defini- 

 tive jaw are arranged. 



"The hyoid arch lies between the spiracle and the first true gill cleft, 

 in the region of the seventh nerve. It divides into an upper element, the 

 hyomandibular cartilage, and a ventral portion, the hyoid proper, which 

 may subdivide into several parts. In the lower elasmobranchs the 

 hyomandibular and the rest of the hyoid arch are closely connected, but 

 in the higher fishes the hyomandibular becomes more separated from 

 the ventral portion and tends to intervene between the mandibular arch 

 and the cranium, becoming a suspensor of the jaws. Still higher it loses 

 its suspensorial functions, becomes greatly reduced, and apparently is 



