FISHES 269 



conical snout, and the other a broad flattened one, as is indicated 

 by the name of Paddle-Fish or Spoon-bill Sturgeon {Spatularia). 

 It is found in the Mississippi. 



The last remaining recent Ganoid, the Bow- Fin {Amid), 

 resembles the ordinary bony fishes in many respects, and was 

 for long confused with them. Its body is covered by thin over- 

 lapping flexible scales. In distribution it corresponds to the 

 Gar- Pike. 



One structural feature in which all these Ganoids differ from 

 the Dog-Fish is the possession of an elongated air-containing sac, 

 the swim -bladder or air-bladder, which opens into the pharynx, 

 usually on the upper side. In the Bichir, however, the air- 

 bladder, like the lungs of terrestrial Vertebrates, is paired, and 

 opens into the under side of that region. The original use of 

 the swim-bladder is apparently that of a balancing organ, but 

 in some Ganoids (Bichir, Gar- Pike, Bow- Fin) it assists in 

 breathing, and in the Dipnoi has been converted into a lung. 

 It is highly probable that the lungs of the higher Vertebrates 

 are simply to be regarded as modified swim-bladders, and if so, 

 an extremely interesting example of change of function is afforded. 



Order 2. — Ordinary Bony Fishes (Teleostei) 



Here are included the ordinary bony fishes of the present 

 day, which include some three thousand existing species. The 

 most important features of the group may be understood by 

 briefly describing an average form, such as the common fresh- 

 water Perch, and noting the differences between it and the 

 Dog-Fish (see pp. 257-264). 



The Common Perch {Perca fiuviatilis) (fig. 163) is widely 

 distributed through the fresh waters of Europe and North Asia, 

 and is very beautifully coloured. The upper parts are of a warm 

 reddish-brown, passing into golden on the sides and again into 

 white below. Several broad dark bands run across the body, 

 tapering off on the sides. Well-grown specimens in this country 

 attain a length of 9 or 10 inches, or even a foot, but much 

 larger specimens have been recorded. 



The body is much flattened from side to side and the outline 

 is spindle-shaped. It has been compared to a rounded wedge, 

 eminently adapted for rapid progression through the water by 



