640 PHYLUM CHORDATA : CLASS PISCES — ^FISHES 



In Amia, the bow-fin of the rivers and lakes of N. 

 America, the scales are cycloid. The male is generally 

 only about two-thirds the length of the female (2 ft.). 

 The skeleton is bony. There is no spiracle. In the heart 

 the conus is contractile only for a short distance at its 

 hinder end, foreshadowing the Teleostean condition. 

 The air-bladder is physostomatous and functions as a 

 breathing organ, being supplied with blood by pulmonary 

 arteries. There are no pyloric caeca, but a vestige of 

 a spiral valve is present at the lower end of the intestine. 

 Amia is a greedy carnivorous fish, feeding on fishes, 

 crustaceans, etc., and frequently rising to the surface to 

 gulp in air. In the breeding season it makes a rude 

 shallow water nest, where the oval eggs, 2-5 x 3 mm. in 

 size, are deposited. Segmentation approaches the mero- 

 blastic type. The eggs rapidly develop into larvae a few 

 millimetres in length, with cement-organs. During 

 development, and for a time after hatching, the male fish 

 remains on guard. The Holostei are of much interest 

 as leading up to the Teleostean type of fishes. 



Order 4. Teleostei. The " Bony Fishes " 



This order includes most of the fishes now alive. 

 Though comparatively modern fishes, they are older than 

 was formerly supposed, as several Jurassic genera {Thrissops, 

 Leptolepis, etc.), which used to be classed as " Ganoids," 

 must be considered as actual Clupeoids, or herring-like 

 Teleostei. It is, however, not until the Upper Cretaceous 

 and Tertiary epochs that they assume among fishes that 

 overwhelming preponderance in numbers which they 

 possess at the present day. The physostomatous type of 

 Teleostean is the most ancient, and probably stands in a 

 continuous genetic line with the Holostei. 



The skeleton is well ossified, with numerous investing 

 bones on the skull, others in the operculum, and on the 

 shoulder-girdle. There is always a supra-occipital in 

 the skull. 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 the young forms is 



