ix. 7 SPINES AND POISON GLANDS 253 



become developed to form a bony armour even more complete than 

 that of the Palaeozoic fishes. Thus in the trunk- or the coffer-fishes 

 (Ostracion) the scales are enlarged and thickened into a rigid box, 

 from which only the pectoral fins and tail emerge as movable struc- 

 tures, the former apparently assisting the respiration, the latter the 

 swimming. These fishes live on the bottom of coral pools and have 

 a narrow beak with which they browse on the polyps. 



6. Spines and poison glands 



Sharp protective spines are often found in teleosts, especially on the 

 operculum and dorsal fins. These may be provided with modified 

 dermal glands that inject poison into the wound. Thus the European 

 weever (Trachinus) lives buried in the sand and has poison spines on 

 the operculum and the dorsal fins. It is suggested that the dark 

 colour of the fins serves as a warning. Some catfishes, scorpion fishes, 

 and toad-fishes also have poison spines. Spines may be effective even 

 if not poisonous; the stargazer, Uranoscopus, of the Mediterranean 

 and tropical waters has powerful spines on the operculum, which 

 inflict a most unpleasant wound if the animal is disturbed by hand 

 or foot while lying in the sand angling for its prey (p. 249). Lophius, 

 the angler, is also armed with dangerous spines. Several species of 

 catfish have large spines, sometimes serrated. In the trigger-fishes 

 (Balistidae and related families) of the tropics one or more of the fins 

 is modified to make a spine that can be raised and locked in that posi- 

 tion. These fishes have very brilliant coloration, but since some of 

 them live in the highly coloured surroundings of coral reefs it cannot 

 be considered certain that the colours serve as a warning. 



7. Electric organs 



The power to produce electric discharges has been developed 

 independently in four distinct families of teleosts, as well as in 

 torpedoes and rays. The electric organs arise bilaterally from modified 

 muscle fibres, the cells of which are plate-like and arranged in rows, 

 the electroplaques. Each plate is innervated on only one surface by 

 motor neurons whose activity is controlled from the forebrain, in 

 some fish there is a controlling nucleus located in the medulla. The 

 physiological properties of transmission at the nerve endings with the 

 electroplaques are similar to those of motor end plates. Unlike other 

 electrogenic tissues such as muscle or nerve, electric organs can 



