. Outlines of zoology. Zoology. 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;
. Outlines of zoology. Zoology. 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 opisthoccel
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
