XIII 



PHYLUM CHORDATA 



233 



fishes (Regcdccus and Trachypterus), the amount of mineral matter 

 is so small that the bones are easily cut with a knife and weigh 

 astonishingly little when dry. 



Electric Organs. Three genera of Teleostomi possess electric 

 organs, the Electric Cat-fish (Malapterurus), one of the Siluridae, 

 found in the fresh waters of tropical Africa, the Electric Eel 

 (Gymnotus), a Physostome occurring in Brazil and the Guyanas, 

 and an American 



Star-gazer of the A 



genus Astroscopes. 

 In Malapterurus the 

 electric organ ex- 

 tends over the whole 

 body, beneath the 

 skin ; in Gymnotus 

 (Fig. 890) there are 

 two pairs of batteries 

 in the ventral half of 

 the greatly elongated 

 tail; in Astroscopus 

 the electric organs 

 are situated on the 

 upper surface of the 

 head just behind the 

 eyes. As in the Elas- 

 mobranchs, the elec- 

 tric organs are formed 

 by modification of 

 muscular tissue. 



Digestive Organs. 

 Some Teleostomi 

 are toothless ; but in 

 most instances teeth 

 are present, and may 

 be developed on the 

 premaxilla, maxilla, 

 palatine, pterygoid, 

 vomer, dentary, basi- 



hyal, and superior and inferior pharyngeal bones. It is character- 

 istic of most Teleostei, with the exception of Physostomi, that the 

 maxilla is edentulous and does not enter into the gape (Fig. 878). 

 In a large majority of species the teeth are small, conical, and 

 recurved, suitable for preventing the struggling prey from slipping 

 out of the mouth, but quite unfitted for either tearing or crushing. 

 In some Fishes, such as the Pike, the teeth are hinged backwards 

 so as to offer no resistance to the passage of the prey towards the 

 gullet, but effectually barring any movement in the other direc- 



FIG. 890. Gymnotus electricus, showing the extent of 

 the electric organ (E). Fl, ventral fin. B, small portion of 

 tail, in section. DM. DM.' dorsal muscles ; E. E.' electric 

 organ ; Fl, ventral fin ; H. skin ; LH, caudal canal ; Sep. 

 fibrous septum ; VM. VM'. ventral muscles ; WS, WS', 

 vertebral column, with spiral nerves. (From Wiedersheim's 

 Comparative Anatomy.) 



