I 



354 ELEMENTS OF BIOLOGY 



ously by the stimulation of the single nerve fibre that communicates 

 with the organ; the voltage of the discharge is the combined volt- 

 ages of the electrical elements in the organ. 



Defense Mechanisms. The electrical discharge of fishes, de- 

 scribed above, is an example of the startlingly effective defensive 

 reactions of animals that have always excited interest and on occa- ■ 

 sion dismay. A recital of the many and extremely varied defenses 

 that animals employ in warding ofif attacks of other animals would 

 be out of place here, but some cases may be examined with profit 

 because they illustrate another of the great variety of possibilities 

 residing in protoplasm. One may consider defensive mechanisms as 

 being either passive or active. Passive defensive mechanisms include 

 protective coloration, spiny coverings, power of escape by rapid 

 locomotion, chemical properties of the organism or its integu- 

 mentary glands, and a multitude of other characteristics that tend 

 to prevent animals from falling victim to carnivorous enemies. One 

 would hardly regard the common garden toad as possessing re- 

 markable powers of self defense, yet its integumentary glands are 

 an example of a passive defense device; they secrete substances that 

 are distinctly poisonous; the toad is rarely bothered by carnivorous 

 animals. 



The electric organs of fishes described in a previous paragraph are 

 active defense organs. As in the case of all weapons, active defensive 

 mechanisms may also be employed on offense by the animal. All are 

 familiar with the stings of insects. The stinging apparatus of the 

 common honey bee is a quite complicated affair, consisting of a pair 

 of sense organs that select the place for insertion of the sting, a 

 hollow shaft or sheath that directs the barbs, and a barbed' dart that 

 is driven outward by suitably placed muscles. At the base of the 

 sheath mechanism are glands which secrete a poison that is injected 

 through the sheath into the site of the sting. Insect bites are usually 

 not the result of active defense but are feeding acts. They involve 

 the piercing of the skin by modified mouth parts. The insertion of 



