212 AMPHIBIA 



they begin to develop at the same time the hind pair are at 

 first alone visible because the anterior pair are concealed by 

 the operculum. The limbs develop directly, first as cylindri- 

 cal outgrowths and then becoming segmented into the parts of 

 the terrestrial limb ; they show no trace in their earlier stages 

 of the structure of the fin of a fish. In the frog-tribe, when 

 the hind limbs are well developed but small, the tail begins to 

 grow smaller, but is not entirely absorbed when the frog leaves 

 the water and becomes terrestrial. In the Urodela the only 

 change which takes place in the tail is the partial or entire dis- 

 appearance of the fin-membrane. Ih newts, which are species 

 of the genus Triton or Molge, the fin-membrane is enlarged every 

 season when the animals resume aquatic habits, and is reduced 

 when they leave the water and lead a terrestrial existence. 

 In Triton the fore-limbs appear much earlier than the hind. 

 In the larvae of Triton vulgaris and other Salamandridae 

 there are a pair of tentacle-like organs situated just above the 

 angle of the jaw on each side. They are little rods developed 

 from the skin and have thickened ends ; they are movable and 

 are used chiefly to prevent the head from sinking into the soft 

 mud at the bottom of the water ; they are known as bal- 

 ancers. These organs occur also in Amblystoma, and in the 

 larvae of the African clawed toad, Xenopus they are long, 

 tapering and conspicuous. It is possible that the retractile 

 tentacles of the Apoda correspond to these balancers. 



The lungs develop as a ventral " out-pocketing " of the 

 throat behind the gill-clefts, dividing posteriorly into two sacs. 

 Each receives an artery from the dorsal part of the fourth arch, 

 and as the gill-clefts close up, the first arch forms the carotid 

 arch sending blood to the head and brain, the second on each 

 side remains connected with the dorsal aorta — the main artery 

 of the trunk — while the third arch disappears altogether. In 

 some cases and under abnormal conditions the transformation is 

 postponed and the animal retains its gills and continues its 

 aquatic respiration throughout life. The most remarkable in- 

 stance of this is the celebrated axolotl. This creature is abun- 

 dant in the lakes near the city of Mexico, it was found there by 

 the Spanish conquerors of the country and was then and is 

 still habitually used as food. The axolotl has three pairs of 

 large external gills and four pairs of gill-clefts, the two pairs of 



