XIII 



PHYLUM CHORDATA 



217 



The distinction between hard or unjointed fin-rays, or spines, 

 and soft or jointed fin-rays has already been referred to. The 

 first ray^of the dorsal and pectoral fins sometimes, e.g. in Siluroids 

 (Fig. 827), has the form of a very strong spine articulated by a 

 bolt-and-shackle joint, i.e. by the interlocking of two rings. In 

 some cases the first dorsal spine springs from the skull. 



The texture of the bones is subject to wide variation : in some 

 Acanthopteri they are very thick and strong, in some places 

 almost like ivory, while in the Lump-fish (Ci/dopterus}, the hug-e 



O 12 1 //I J.7 " 



ounnsh (Ortlw.goms- 

 cus\ and in many 

 deep-sea forms, such 

 as the Ribbon-fishes 

 (Regalecus and Trac- 

 hypterus), 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. 

 Two genera of Teleo- 

 stomi possess electric 

 organs, the Electric 

 Cat-fish (Malapter- 

 urv.s), one of the Sil- 

 uridae, found in the 

 fresh waters of tropi- 

 cal Africa, and the 

 Electric Eel (Gym- 

 iiotns), a Physostome 

 occurring in Brazil 

 and the Guyanas. 

 In Malapterurus the 

 electric organ ex- 

 tends over the whole 

 body, beneath the 

 skin ; in Gymnotus 



(Fig. 840) there are two pairs of batteries in the ventral half of 

 the greatly elongated tail. 



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 (Fig. 829) and does not enter into the gape. 



IT.... 



,. S40. Gymnotas 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 spinal nerves. (From Wiedersheim's 

 Comparative Anatomy.) 



