524 PISCES— FISHES. 



resemble very closely the corresponding stages in the 

 development of Amphibians. 



Ceratodus sometimes attains a length of 6 ft. The body is elon- 

 gated and compressed, and bears a continuous vertical fin. The paired 

 fins are trowel-like, with a median jointed axis, from which rays project 

 on each side. There are four gill-clefts, four internal gills, and a hyoid 

 half-gill. There are no external gills. 



The swim-bladder or lung — for as such it acts — is single. It is sup- 

 plied with blood from the fourth aortic arches, as is the swim-bladder 

 of the Ganoids — Polypterus and Amia. It arises ventrally, but lies 

 dorsally, and is divided into compartments. 



The heart has only one auricle, with a dorsal fibrous ridge hinting at 



sn.t. 



Fig. 225. — Head region of Protopterus. — From W. N. Parker. 



sn.t., Sensory tubes ; /./., lateral line ; e.br., external gills ; pel., 

 pectoral fin ; op., operculum. 



a division. The conus arteriosus is peculiarly twisted, and contains 

 a short longitudinal spiral valve and numerous large "pocket" (or 

 " Ganoid ") valves. The septum in the conus is not complete, as it is 

 in the other Dipnoi, thus mixed blood passes into the first two pairs of 

 arches. There are four pairs of these arches or arteries supplying the 

 gills ; the efferent vessels (two from each gill, as in Elasmobranchs) 

 unite to form epibranchials, and these to form the dorsal aorta. The 

 fourth epibranchial gives off the pulmonary artery. The pulmonary 

 vein enters the left side of the auricle. 



Protopterus. — This mud-fish lives in the Gambia, Quili- 

 mane, and some other African rivers. It is mainly but not 



