43° 



FISHES. 



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 moulih 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 i8 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 Scaphir- 

 hynchus 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 — Lepidosleus — is 

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

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

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

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

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



The fossil Ganoids appear in the Silurian about the same time as the 

 Elasmobranchs, they are abundant from the Devonian to the Upper 

 Cretaceous when the Teleosteans begin to become numerous. According 

 to Traquair, a Proganoid series of primitive forms, such as Pteraspis, 

 Cephalaspis, Pterichlhys, Coccostevs, should be distinguished from the 

 Euganoid series of typical forms, such as Holoptychitts, Rhizodus, 

 Osteolepis, Chondrosieus. 



Order IV. Teleostei — the " Bony Fishes." 



This order includes most of the fishes now alive. As they 

 do not appear before the Cretaceous period, the Teleosteans 

 are relatively modern fishes. They are the successors and 

 perhaps the descendants of the Ganoids. 



General Characters.— T^& skeleton is well ossified, with 

 numerous investing bones on the skull, others in the oper- 

 culum, and on the shoulder girdle. The tail is sometimes 

 quite symmetrical or diphycercal, but in most cases it is 

 heterocercal at first, and acquires a secondary symmetry 

 termed homocercal, for while the end of the notochord in 



