508 FISHES. 



the skull and shoulder girdle. The endoskeleton is in great 

 part cartilaginous in Acipenser^ Scaphirhynchus, and Spatu- 

 laria, but is ossified in Lepidosteus, Polypterus, Calamoichthys^ 

 and Amia. In the first three the notochord is uncon- 

 stricted ; in the others there are distinct vertebral bodies, 

 opisthoccelous in Lepidosteus, amphiccelous 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 urinogenital aperture. There are also abdo- 

 minal pores. An air bladder is present with a persistent 

 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 in the water; they have comparatively 

 little yolk, and so far as we know, their 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 

 asymmetrical or heterocercal ; the notochord is unsegmented. A snout, 

 bearing 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 fishes found in fresh water, for 

 A. sturio may attain a length of 18 feet, and a weight of 600 pounds, 

 while the A. huso of Southern Russia may measure 25 feet, and weigh 

 nearly 3000 pounds ! Most of the species are found both in the sea and 

 in rivers 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 Scaphi- 

 rhynchus is represented in Asia and the United States ; Polyodon or 

 Spatularia spatula is the paddle fish or spoon bill of the Mississippi. 

 In PolypteruS) 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 

 related genus Calamoichthys. The gar pike or bony pike Lepidostetts 

 is covered with rows of enamelled scales ; the whole skeleton is well 

 ossified, and the vertebral bodies are opisthoccelous or concave behind ; 

 the swim bladder is like a lung in structure, and to a slight extent in 

 function. The bow fin, Amia calva, frequenting still waters in the 

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



