TELEOSTOMI 219 



a billion and a half. Efforts are made to rid the streams of 

 voracious fishes, such as the pike, pickerel, and muscalonge, 

 which feed upon our food fishes. 



Besides being used for food, fishes furnish other useful prod- 

 ucts. The skin of the "dog-fish" (shark) is used as leather, and 

 shagreen. The bodies are used as guano or fertilizer. Oil is 

 obtained from the menhaden, cod, and other forms. Caviare 

 is a preparation of the salted roe of sturgeons, the preparation of 

 which constitutes an important industry on the Black and Cas- 

 pian Seas. Scales of some species are used in ornamental work, 

 and the teeth of sharks are used as weapons by Pacific Islanders. 

 The swim-bladders of cod-fishes are used in making isinglass. 

 They are also pickled and eaten under the name of "sounds." 



Geologic Distribution. — Teeth of the true fishes have been 

 found in the Ordovician of Europe. The remains of sharks prove 

 their existence in the Silurian. Fishes are found in great variety 

 and abundance in the Devonian Period, the sharks, lung-fishes, 

 Crossopterygii and the Ganoids, the most advanced, are repre- 

 sented. The bony fishes (Teleosts) are entirely absent in the 

 Devonian. These, according to Scott, are approximated by 

 some of the Jurassic fishes. In the Cretaceous Period, Ganoids 

 become rare and Teleosts take the dominant place among fishes. 

 Marine and fresh-water fishes assume the modern forms in the 

 Eocene Epoch. 



Important Biologic Facts. — The skull is a continuation of 

 the vertebral column, and contains, but is not filled, by a genu- 

 ine brain. The vertebrae are amphicoelous, that is, concave at 

 each end. The true fishes have true jaws. 



They have a closed, though an incomplete circulation. 

 The blood-corpuscles are red. 



The multiplication is sexual, but the eggs, or roe, are ferti- 

 lized in the water. 



The skin of vertebrates is distinguished from that of inverte- 

 brates by the many layered condition of the epidermis and the 

 thickness of the dermis. The scales of fishes are of dermal origin 

 and different from the epidermal scales of reptiles. It is 

 from the dermal scales that the bony plates of turtles and 

 armadillos have arisen, as well as the secondary or membrane 

 bones. 



