154 MORPHOLOGY OF THE ORGAXS OP 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- 



FiG. 162. EKagram of skull and visceral arches of an Elasmobranch. a, audi- 

 tory capsule ; b, basibranchial ; c , keratobrancbial ; d, epibranchial ; g, gill cleft ; h, 

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

 o, olfactory capsule; /, pharyngobranchial ; /^, pterygoquadratr ; r, rostrum; j, 

 spiracle; 2, 5, exits of second and fifth nerves: I—V, branchial arches. 



pletely surround the alimentary canal, were compared more or 

 less closely with the ribs, but that this homolog}" cannot be held 



is shown by the fact that the ribs 

 develop from the somatic mesen- 

 ch)Tne (j.e., that outside the coelom), 

 while the \nsceral skeleton arises 

 from the splanchnic mesench}Tne. 

 This visceral skeleton is seen in its 

 simplest condition in the region of 

 the gill clefts (p. 2j), where there is 

 developed a branchial cartilage, a 

 rod-like structure, between each two 

 successive gill slits. In their sirr.- 

 plest condition these are simple rods. 

 but usually they become broken up 

 into a series of elements, t\-pically 

 four in 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 



"Fig. 163. Visceril arches of 

 ScrHiiim, after Gegenbaur. BH, 

 "basihyal ; C, copula (united basi- 

 Ijranchials) ; E, epihyal ; H, hypo- 

 lyal; HM, hyomandibniar ; HV, 

 hyoid ; K. keratobranchial ; P, 

 pharyngobranchial. 



