GANOIDEI. 519 



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 Mullerian ducts are formed; the 

 pronephros completely degenerates. The ova are small, 

 and are fertilised in the water ; they have comparatively 

 little yolk, and, so far as we know, the segmentation is 

 holoblastic. 



Genera. — The sturgeon {Acipenser) is one of the more cartilaginous 

 Ganoids. The skin bears five rows of large bony scutes ; the tail is 

 heterocercal ; the notochord is unsegmented. A snout, with pendent 

 barbules, extends in front of the ventral mouth, which is rounded and 

 toothless. Sturgeons feed on other fishes, which they swallow whole. 

 They are the largest fresh-water fishes, for A. sturio may attain a length 



Fig. 223. — Pterichthys Milleri. Lateral view. — Restored 

 by Traquair. 



of 18 ft., and a weight ot 600 lb., while the A. huso of Southern 

 Russia may measure 25 ft., and weigh nearly 3000 lb. ! Most of 

 the species are found both in the sea and in riveis or lakes. The flesh 

 is edible, except in the case of the green sturgeon, A. medirostris , of 

 the Pacific coasts, which is said to be poisonous. The roes or ovaries 

 form caviare ; the gelatinous internal layer of the swim-bladder is used 

 as isinglass. 



The genus Scaphirhynchus is represented in Asia and the United 

 States ; Polyodon or Spatularia spatula is the paddle-fish or spoon-bill 

 of the Mississippi. In Palyptems, from the Nile and other African 

 rivers, the dorsal fin is divided into many parts ; the nasal-sac has 

 a complex labyrinthine structure ; the swim-bladder arises from the 

 ventral side of the gullet ; the young are said to have external gills. In 

 Old Calabar there is a 1 elated genus, Calamoichthys. The gar pike 

 or bony pike — Lepidosteus — is covered with rows of enamelled scales ; 

 the whole skeleton is well ossified, and the vertebral bodies are 

 opisthoccelous ; the swim-bladder is like a lung in structure, and to 

 some degree in function. The bow fin, Amia calva, frequenting still 

 waters in the United States, has a similar lung-like swim-bladder. 



