A HISTORY OF VERTEBRATES: AMPHIBIANS AND REPTILES 449 



of Streams, and some are entirely aquatic. A rather generalized type is 

 Jefferson's salamander, Amby stoma jeffersonianum (Fig. 23.2), of the 

 eastern United States. This species is terrestrial as an adult, but returns 

 to the water in early spring to reproduce. Breeding is sometimes pre- 

 ceded by a nuptial dance in which many individuals writhe about in 

 the water, rubbing and nosing one another. The males deposit sperm 

 in clumps called spermatophores on sticks and leaves in the water. Later 

 the females pick these up with their cloacal lips. Fertilization is internal, 

 and the fertilized eggs are deposited in masses attached to sticks in the 

 water. The larvae of salamanders differ from those of frogs and toads in 

 retaining external gills throughout their larval life, and in having true 

 rather than horny teeth. 



There have been many special adaptations among salamanders. The 

 most abundant of our American species are woodland types like the 

 red-backed salamander {Plethodon cinereus), which belongs to the family 

 Plethodontidae. A particularly interesting feature of plethodonts is their 

 complete loss of lungs; gas exchange occurs entirely across the moist 

 membranes lining the mouth and pharynx and the skin. The skin is a 

 more effective respiratory organ than in frogs because the epidermis is 

 very thin and capillaries come close to the surface. Loss of lungs may 

 seem to be a curious adaptation for a terrestrial vertebrate, but it has 

 been postulated that early in their evolution plethodonts became 

 adapted for life in rapid mountain streams. Air in the lungs would be 

 disadvantageous under these conditions, for the animals would float and 

 be washed away. Lungs may have been lost in adapting to this habitat. 

 Subsequently jjlethodonts may have entered different environments, but 

 never regained the lost lungs. 



Figure 23.2. Jefferson's salamander, Ambystoma jeffersonianum, reproduces in the 

 water. The four white structures attached to sticks are spermatophores. A clump of 

 eggs can be seen in the upper righthand corner. 



