x GANOID FISHES 371 



resembling in general appearance those of Polypterus, while in Amia tin- 

 scales have become cycloid. A physostomatous air-bladder is present 

 but an interesting relic of its ancestral history is seen in the fad that in 



FIG. 156. 

 Acipcnser. x ,V (From Bashford Dean.) 



Amia it still receives its blood from typical pulmonary arteries, though 

 it is to be noted that their arrangement has become modified in the 

 same direction as was seen in the nerves of the right lung of Polypterus 



FIG. 157. 

 Lepidosteus. x $. a./, Anal fin ; d, dorsal fin ; op, opercular opening ; pl.f, pelvic fin. 



the left pulmonary artery passing direct to the left side of the air-bladder 

 and the right artery direct to its right side. 



In the Sturgeons an open spiracle is still present but it has disappeared 



Amia. X f. 

 pl.f, pelvic fin. 



FIG. 158. 

 i./, Anal fin d, dorsal fin ; olf.i and 2, olfactory openings ; op, opercular opening 



in the other actinopterygian ganoids. The post-spiracular gill-clefts 

 are covered in by an operculum and there are no external gills at any 

 period of development. As regards the rest of the alimentary canal 

 there is in the Sturgeons a well-developed spiral valve but in Amia and 



