XIII 



PHYLUM CHORDATA 



217 



The distinction between hard or unjoiiited 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 (Cyclopterus), the huge 

 Sunfish (Orthagoris- A 



cm), and in many 

 deep-sea forms, such 

 -as the Ribbon-fishes 

 (Eegakcus and Trac- 

 hyptcrus), 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- 

 urus), one of the Sil- 

 uridse, found in the 

 fresh waters of tropi- 

 cal Africa, and the 

 Electric Eel (Gym- 

 notus), 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. 



Ft.. 



Via. 840. G 



ymnotas 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 Wiedersneim s 

 Comparative Anatomy.) 



