ADAPTATION 331 



potent and kills a small animal in a few minutes. It may also 

 cause the death of a human being. Rattlesnakes are not immune 

 to their own poison. In captivity they sometimes bite them- 

 selves and die as a result. On the other hand, the rattlesnake 

 suffers no ill effects from eating prey killed by its venom. The 

 rattle consists of a series of cornified shells on the end of the 

 tail which produce a shrill note when vibrated. This "warning 

 signal" is a protective adaptation for the snake, since its use in 

 most cases serves to keep its enemies at a distance. The rattle 

 itself is harmless and receives an additional segment after each 

 molt. Since molting occurs at irregular intervals the number of 

 rattles is not an exact indication of the snake's age. 



The skin secretion of some amphibians is highly poisonous in 

 nature. Various species of Bufo (toads), which are otherwise 

 defenseless, produce in their skin highly toxic substances, which 

 render them inedible. One of these substances is bufotalin, 

 which has the formula C34H46O10. Another is bufogin, C 18 H 2 40 4 . 

 Both these substances when tested on experimental animals have 

 an action similar to digitalis. They stimulate the heart muscle, 

 eventually stopping its action. The secretions of the parotoid 

 glands of the skin in the region of the ear and the side of the body 

 have been found to contain adrenalin, which in other animals 

 is a secretion of the adrenal gland. The secretions of a tropical 

 toad, Dendrobates, is said to be used by the Indians of Colombia 

 as a source of poison for their arrows. 



The electric organ of certain fishes is a rather remarkable 

 example of an organ that is capable of producing an electric 

 current, which in the case of the electric skate, Torpedo, or the 

 electric eel, Gymnotes, is great enough to be considered of value 

 as a means of protection. The electric organ is built on the 

 general principle of a voltaic pile. A voltaic pile consists of a 

 column of alternating disks of dissimilar metals, such as copper 

 and zinc, separated by pieces of cloth or paper moistened with 

 dilute acid. When the top and bottom disks, or any two neigh- 

 boring disks, are connected by a wire, an electric current flows 

 from the zinc disk to the copper disk. In Torpedo marmoraia, a 

 Mediterannean skate, the electric organ consists of rows of 

 electroplaxes (Fig. 187) forming a considerable mass of tissue on 

 either side of the head. Each electroplax consists of an inner 

 striated layer covered above and below by a gelatinous layer. 



