86 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



possible that the opercular bones have been developed in connexion with the 

 hyomandibular rays of the elasmobranchs. There are five branchial arches, 

 the last more or less reduced. Often they bear teeth on their inner surfaces, thus 

 acting as accessory chewing organs. 



Fig. 86. — Chondrocranium of Polypterus, after Budgett. a, afferent artery to 

 external gills; b^~*, branchials; ch, ceratohyal; hh, hyophyal; hm, hyomandibular; lb, 

 labial; Mk, Meckel's cartilage; op, operculum; pq, pterygoquadrate; sh, stylohyal; 

 2, 5, 7, nerve exists. 



The chondrostei, the most shark-like of the Ganoids, have no cranial 

 cartilage bones. They are also primitive in the great development of the rostral 

 cartilage (enormous in Polyodon), which gives the mouth its ventral position, 

 and in the extension of the cranial cavity into the ethmoid region. They have 

 a few bones in the visceral skeleton, while there are numerous membrane bones 



Fig. 87. — Median section of skull of mackerel {Scomber) after Allis. For letters see 



fig. 85. 



in the roof of the skull, a few of them readily homologized with those of other 

 vertebrates. 



In other ganoids (holosteans and crossopterygians) the skull is much like 

 that of the teleosts, differing in the extension forward of the cranial cavity. 

 There are one (Amia) or two (Polypterus) gular bones developed between the 

 rami of the lower jaw, and in Polypterus parietals, frontals and nasals fuse with 



