154 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



The visceral skeleton consists of a series of paired bars, 

 always preformed in cartilage, in the walls of the pharynx and 

 the oral cavity. Formerly these arches, which partially or com- 



iv b 



t BH 



HM 



FlG. 162. Diagram of skull and visceral arches of an Elasmobranch. a, audi- 

 tory capsule ; b, basibranchial ; c , keratobranchial ; <?, epibranchial ; g, gill cleft ; h, 

 hyoid; km, hyomandibular ; /, labial cartilages; m, mandible (Meckel's cartilage); 

 o, olfactory capsule ; /, pharyngobranchial ; pq, pterygoquadrate ; r, rostrum ; s, 

 spiracle; 2, 5, exits of second and fifth nerves; IV, branchial arches. 



pletely surround the alimentary canal, were compared more or 

 less closely with the ribs, but that this homology cannot be held 



is shown by the fact that the ribs 

 develop from the somatic mesen- 

 chyme (z>., that outside the coelom), 

 while the visceral skeleton arises 

 from the splanchnic mesenchyme. 

 This visceral skeleton is seen in its 

 simplest condition in the region of 

 the gill clefts (p. 22), where there is 

 developed a branchial cartilage, a 

 rod-like structure, between each two 

 successive gill slits. In their sim- 

 FIG. 163. Visceral arches of p j est condition these are simple rods, 



Sf y Ilium, after Gegenbaur. Bff, , ,11 



tosihyal; C, copula (united basi- but USUall y the y become broken up 



"branchiais) ; E, epihyai; //, hypo- into a series of elements, typically 

 l y al; HM, hyomandibular; HY, f our j n number, movably articulated 



with each other, and named, pro- 

 ceeding from above downwards, 

 pharyngobranchial, epibranchial, keratobranchial, and hypobran- 

 chial. Between the two hypobranchials of each arch is devel- 

 oped an unpaired piece, the copula or basibranchial, and these 



liyoid ; K, keratobranchial ; P, 

 pharyngobranchial . 



