XIII 



rilVLl.M CIIORDATA 



t < 



bars booonK^ united with one another hv\o\\ by an unpaired cartilage 

 (Fi,^-. 7()4, A, b. hr.), Wmnm^n visceral arch, and tlic unpaired ventral 

 pieces may unite successive arches with one another in the middle 

 ventral line, thus giving rise to a more or less basket-like visrcral 

 sJccletun. It will be noticed that the visceral skeleton has a seg- 

 mental arrang(unent, being i'ormcd of parts arranged in an antero- 

 posterior st'ries, whereas in the cranium there is no clear indication 

 of segmentation. There is, however, no exact corres})(jndence 

 between the segments of the visceral skeleton and the metameres. 

 The visceral arches \aiy in number from four to nine: the fore- 



m.cth 



>r >L,j i. ;.. ....'^ ;;V.V _ "^ 



^ — / ' ■ """' "\^ n.ch. 



olf.ap 



j/.t b.cr- 

 nv.si 71,1/ s 



n/t/.io 

 ccu,cp 



A.Ay 



Fif!. '(frt.—A, diagram of cartilaginous skull from Uie left side; B, cranium in sagittal section. 

 au. cp. auditory capsule ; h. In: l—'i, basi-branchials ; //. i-r. basis cranii ; //. /(//. basi-hyal ; 

 c. hr. cerato-brandiial ; r. hy. cerato-hyal; <■//. hr. eiii-l.irauchial ; ep. hy. epi-hyal ;./"o/i. fontancUe; 

 for. mwi. foramen magnum ; h. hr. hypo-branchial ; /<. hi/, hypo-hyal ; hy. m. hyomandibular ; 

 Ih. l—U, labial cartilages ; larl-.'c. Meckel's cartilage ; m. tth. mesethmoiil ; nv. 1—10, foramina 

 for eerel)ral nerves; olf. cp. olfactory capsule; paLqu. palato-(£uadratc ; ph. hr. pharyngo- 

 branchial ; /■. rostrum ; s. t. pituitai-y fossa or sella turcica. 



most of them is distinguished as the mmidibular arch, and lies 

 just behind the mouth ; the second is called the h'l/oid 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 culled jaws for the support of the 

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

 portion called respectively the palato-quadrate cartilage (Fig. 764, 

 A,pnl. qu.) and Meckel's cartilacjc {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 iLiyper 

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



