xnr 



PHYLUM CHORDATA 



77 



bars become united with one another below by an unpaired cartilage 

 (Fig. 764, A, b. br.), forming a visceral arch, and the 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 visceral 

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

 mental arrangement, being formed of parts arranged in an antero- 

 posterior series, whereas in the cranium there is no clear indication 

 of segmentation. There is, however, no exact correspondence 

 between the segments of the visceral skeleton and the metameres. 

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



B 



b.br.s 



Fin. Y64. A, diagram of cartilaginous skull from the left side ; B, cranium in sagittal section. 

 a-u. cp. auditory capsule ; b. br. 1 5, basi-branchials ; b. cr. basis cranii ; b. hy. basi-hyal ; 

 c. br. cerato-branchial ; c. hy. cerato-hyal; ep. br. epi branchial ; ep. hy. epi-hyal ;fon. fontanelle; 

 for. mat/, foramen magnum ; A. br. hypo-branchial ; A. hy. hypo-hyal ; Aw. m. hyomandibular ; 

 lb. 1 U, labial cartilages ; 'nu:k."c. Meckel's cartilage ; m. eth. mesethmoid ; nv. 110, foramina 

 for cerebral nerves ; olf. cp. olfactory capsule ; pat. qu. palato-quadrate ; ph. br. pharyngo- 

 branchial ; r. rostrum ; s, t. pituitary fossa or sella turcica. 



most 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-quadrate cartilage (Fig. 764, 

 A, pal. qu.) and Meckel's 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 

 j<>- : Meckel's cartilages similarly extend along the lower or 



