CHAPTER VII 



TRITON : A CHORDATE WITH 5-TOED LIMBS 



Externals. — Triton, the newt, is sharply distinguished from 

 all the types so far described, because its limbs end in fingers 

 and toes instead of being fins. The foot has five toes, but in 

 the newt and allied animals, the number of fingers on the hand 

 has been reduced from five to four. Triton and all higher 

 vertebrates are typically land-animals, and are collectively 

 called the Tetrapoda. Some of them, however, have reverted 

 to the condition of living in water, in varying degrees. So the 

 whales, seals, and hosts of extinct marine reptiles have come to 

 live almost if not entirely in water, and the newt also spends 

 more of its time in water and is more adapted to it than its 

 ancestors were. This secondary return to aquatic conditions 

 is, however, easily and fundamentally distinguished from the 

 primitive aquatic habit of the fish. The possession of typically 

 5-fingered, " pentadactyl," limbs is a sure criterion of a 

 terrestrial animal, or of one whose ancestors were terrestrial. 

 As an example of the secondary readaptation to aquatic 

 conditions may be mentioned the webs of skin which in some 

 species of newts extend between the fingers, and are used for 

 swimming. 



The skin is soft and slimy owing to the presence of glands, 

 and is used largely as a respiratory surface for the oxygenation 

 of the blood. There are no scales or fin-rays of any sort. 

 The tail carries a continuous median dorsal and ventral fin, 

 and in the male animals of some species, there is also a fin 

 along the back, which becomes enlarged at the breeding season. 



The first part of the life is spent in the water in which the 

 eggs are laid and hatched, and since the early stages are aquatic 



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