156 MORPHOLOGY OF THE ORGAXS OF VERTEBRATES. 



Each branchial arch may develop cartilaginous outgrowths — 

 the branchial rays — which serve as supports for the gills. 

 These may also occur upon the hyomandibular arch in those 

 forms where a spiracular gill is developed ; but in the teleosts 

 the rays of the hyoid portion of the arch are modified into 

 slender bony rods — the branchiostegal rays — which support a 

 membrane closing in the gills beneath ; while the rays of the 

 hyomandibular are represented by the opercular bones to be 

 described below. 



The most anterior of the visceral arches, the mandibular 

 arch, has lost all connection with the respiratory region, and 

 has divided into two portions, which are bent on each other so 

 that they meet at a sharp angle behind. The upper of these is 

 the pterygoquadrate,^ the lower is Meckel's cartilage. These two 

 cartilages of the two sides form the jaws in the elasmobranchs. 

 Both pter}'goquadrate and ^Meckel's cartilage frequently have 

 accessory labial cartilages developed in connection with them. 

 These have been interpreted as degenerate arches in front of 

 the mandibular arch. 



The foregoing outline of the cartilaginous skull applies to 

 the gnathostome forms ; but before going more into detail, a 



G 



Fig. 165. Cranium and branchial basket of Petromyzon, after W. K. Parker. 

 B, branchial basket ; E, otic capsule ; G, gill slits ; X, nasal capsule ; XT, noto- 

 chord. 



word must be said concerning the cyclostomes. The chondro- 

 cranium is formed of parachordals, otic capsules, and trabeculas, 

 the cranial cavity being partially roofed in by a narrow tegmen, 

 the so-called occipital arch. In front the cranium is closed by 

 a cartilaginous nasal capsule. The branchial skeleton consists 



1 The term palatoquadrate sometimes applied to this is a misnomer, since the palatine 

 bone is a membrane bone. 



