GANOIDEI. 429 



From the ventral aorta, alike in Cevatodtis and Protofterus, four pairs 

 of branchial arteries arise. In Ceratodus, the two anterior pairs carry 

 mixed blood ; in Pi otopterus, they carry pure blood — a difference due 

 to the imperfection of the partition in the conus of Ceratodus. Only in 

 Ceratodus, moreover, are there internal gills corresponding to the first 

 two pairs of branchial arteries, though the first external gill of Pro- 

 topterus is supplied from the second branchial artery, as are the following 

 two from the third and fourth. The efferent branchial arteries form a 

 dorsal aorta as usual, but from the fourth efferent branchial arises the 

 pulmonary artery. Thus the blood which returns from the lung to the 

 left auricle has been twice purified. 



That the Dipnoi are ancient may be inferred from their structure 

 and distribution. Ceradotus was represented in the Permian (?) and 

 Triassic, and related extinct forms appeared much earlier, such as 

 Dipterus in the Devonian, Ctenodus from Carboniferous to Permian. 



The affinities of Dipnoi and Amphibians are very close, especially as 

 regards the respiratory and the circulatory systems. Indeed, Dipnoi 

 have been called " perennially aquatic Amphibians." 



Order III. Ganoidei. 



This ancient order of armoured fishes flourished in 

 Devonian and Carboniferous ages, but is now represented 

 by only seven genera, of which the Sturgeon {Acipenser) and 

 the Bony Pike (Lepidosteus) are the most familiar. 



General Characters. — The skin bears large scales, or bony 

 scutes. The tail is either heterocercal or homocercal. 

 Membrane bones invest the skull and shoulder girdle. The 

 endoskeleton is in great part cartilaginous in Adpenser, 

 Scaphirhynchus, and Spatularia, but is ossified in Lepidosteus, 

 Folypterus, Calamoichthys, and Amta. In the first three, the 

 notochord is unconstricted ; in the others there are distinct 

 vertebral bodies, opisthoccelous in Lepidosteus, amphicoelous 

 in the other three genera. The fore-brain has a non-nervous 

 roof. There is a spiral valve in the intestine, but it is very 

 small in Lepidosteus. The food-canal ends apart from and 

 in front of the urinog^nital aperture. There are also 

 abdominal pores. An air-bladder is present with a persist- 

 ent open duct. The openings of the gill-clefts are covered 

 by an operculum supported by bones ; in some of the genera 

 there is a spiracle. A conus arteriosus is associated with 

 the ventricle. The archinephric or segmental ducts do not 

 divide; thus no Miillerian ducts are formed; the pronephros 

 completely degenerates. The ova are small, and are fertilised 



