THE ORGANISM AND ITS ENVIRONMENT 353 



tomed to it and adds more gas to the bladder contents to compen- 

 sate for the extra weight. Now if the weight be suddenly removed, 

 the gas in the bladder is in excess of requirement and the fish 

 comes rapidly to the surface and really has quite a struggle to 

 remain under water until the excess gas has been released and the 

 buoyancy returned to normal. In some varieties of fish the gas 

 bladder has no tubular outlet and excess gas is returned to the 

 blood by a diffusion mechanism. In others the bladder is connected 

 to the pharynx by a small tube somewhat like that of the dipnoids 

 (p. 170) and excess gas is released through this tube. 



Electric Organs. Certain fishes are capable of discharging an 

 electric shock amounting in extreme cases to two or three hundred 

 volts, well over twice the voltage of an ordinary house current. The 

 most common electric fishes are an African catfish, a South Amer- 

 ican eel, and a skate (elasmobranch) occurring in the coastal waters 

 of the Gulf of Mexico and the Atlantic Ocean north as far as 

 Cape Cod. 



The electric organs consist essentially of great numbers of sheets 

 of tissue superimposed to form electrical piles. All the sheets receive 

 a branch from the same nerve fibre, and each sheet represents a 

 greatly modified muscle. We know already (p. 234) that a stimulus 

 is accompanied by a discharge of electricity; this is especially 

 marked in the contraction of a muscle. Thus the contraction of any 

 ordinary muscle, for example in the human arm, is accompanied 

 by an electrical discharge of minute voltage and recovery from con- 

 traction includes the restoration of the original electrical charge on 

 the boundaries of the muscle cells. The electrical organs of fishes 

 are devices that take advantage of the electrical character of muscle 

 action and add together the electrical properties of many sheets of 

 muscle-like tissue; so the voltage varies with the number of sheets 

 that compose the organ and with the stage of fatigue. When the 

 organ is in normal resting condition and the animal is stimulated 

 the electrical charge on the numerous sheets is released simultane- 



