The Tctrapoda: Amphibia, Reptiles, Birds 597 



BIOLOGY OF THE AMPHIBIANS 



The life history of Rana pipicns as discussed previously is quite 

 typical of that of most of the amphibians. In this life history, the 

 eggs were laid in water and they hatched into tailed larvae with external 

 gills. During metamorphosis, the tail and gills are lost and the adult 

 form is gradually attained. This adult is a terrestrial form which is car- 

 nivorous in its food habits. It is, however, forced to return to water to 

 lay its eggs. 



While this life history can be considered as the usual type, there 

 are many variations of it as the different species respond to different 

 factors in their environments. 



Urodela. — One of the commonest of the salamanders through much 

 of the United States is the tiger salamander, Anibystoma tigrinum (Fig. 

 202,a). This large (6 to 9 inches long) spotted salamander spends 

 most of its life buried in the moist mud. In the spring they emerge in 

 large numbers and migrate to pools. Here a complex mating dance 

 occurs during which the males deposit bundles of sperm (sperma- 

 tophores). These are picked up by the lips of the females' cloacas and 

 internal fertilization follows. The eggs are laid in masses surrounded 

 by a jelly envelope and attached to twigs in the water. The time re- 

 quired for hatching varies with the temperature, but usually takes 24 

 to 30 days. At this time the gilled larvae which are about 14 mm. long 

 emerge. During the summer, these larvae metamorphose into the typical 

 adult form. 



It has been shown that the thyroid gland is responsible for the trans- 

 formation of these larvae into adults. In some areas where there is little 

 or no iodine in the water, the thyrokl gland does not function properly, 

 and metamorphosis does not occur. These animals remain permanently 

 as larvae, but are able to reproduce. They are known as axolotls. If 

 iodine is administered to these axolotls, they metamorphose into the 

 usual adult type. They are very common in many of our plains states 

 and in the Valley of Mexico where they are at times used as food. 



In the United States there are many other species of Aiubystoiiia, 

 including the silver-barred Ambystouia opaciim which is more terrestrial 

 in its habits. Its eggs are deposited on land in the fall. 



The members of one family, the Plethodontidae, have secondarily 

 lost their lungs and rely entirely on their thin skins for respiration. 

 Species of these remarkable salamanders occur throughout the United 

 States, southern Canada, and the New World tropics. Many are hand- 



