VERTEBRATA : FISHES. 



491 



to form a fringe round a central lobe (fig. 265). This structure 

 characterises a large and important division of Ganoid fishes, 

 called by Professor Huxley, for this reason, " Fringe-finned " 

 Ganoids or Crossopterygidcz. The same form of fin is seen in 



Fig. 265. Ganoid Fishes. A, Polypterus; B, Osteolepis (extinct), a One of the pec- 

 toral fins, showing the fin-rays arranged round a central lobe ; b One of the ventral 

 fins ; c Anal fin ; d Dorsal fin ; d' Second dorsal fin. 



Ceratodus among the Dipnoi, in which the limb (fig. 264) con- 

 sists of a median cartilaginous axis, formed by a succession of 

 joints, which, in turn, support on each side a lateral series of 

 jointed branches, these finally bearing the fin-rays. The form 

 of the caudal fin varies, the Ganoids being in this respect 

 intermediate between the Bony Fishes, in which the tail is 

 " homocercal," and the Sharks and Rays, in which there is a 

 " heterocercal " caudal fin. In the majority of Ganoids, then, 

 the tail is unsymmetrical or " heterocercal," but it is sometimes 

 equi-lobed or " homocercal." 



IV. As to the structure of the respiratory organs, the Ganoid 

 Fishes agree essentially with the Bony Fishes. They all possess 

 free pectinated gills attached to branchial arches, and enclosed 

 in a branchial chamber, which is protected by an operculum, 

 and is closed by a branchiostegal membrane, usually supported 

 by branchiostegal rays. Besides the ordinary branchiae there 

 is frequently an additional gill, called the " opercular branchia," 

 attached to the interior of each operculum, and below this a 

 false gill or "pseudo -branchia," which receives arterialised 

 blood only. Adpenser and Polypterus have " spiracles " placed 

 on the top of the head and communicating with the mouth. 



