CHAPTER V 



VARIATION AND ADAPTATION 



Metamorphosis and occasional persistence of the larval condition (Neoteny). 

 History of the axolotl and its explanation. Colour : protective and warning 

 coloration. Adaptations for locomotion. Adhesive discs of tree-frogs. The 

 spade of Pelobates. Respiratory adaptations : salamanders with neither lungs 

 nor gills. Allantoic gills in embryos. Adaptations in the male for pairing. Con- 

 vergent evolution in tree-frogs. 



IN the commoner species, such as newts, frogs and toads, 

 the young animal is hatched in the water as a larva, 

 known as a tadpole. At the time of hatching the tail 

 is still short, the mouth is indicated by a rhomboidal de- 

 pression, but it has no opening, the anus is formed, there are 

 branchial or gill-arches visible as transverse ridges, but no gill- 

 clefts. On the first and second branchial arches on each side 

 are small branched external gills. On the ventral surface of 

 the head behind the mouth is a transverse crescentic groove 

 bounded by ridges ; this is the adhesive organ commonly but 

 erroneously called the sucker ; it has no muscles of suctorial ac- 

 tion, but consists of cutaneous or skin glands, producing a sticky 

 secretion which enables the larva to attach itself to the surface 

 of leaves or other objects. After hatching it divides into two 

 separate parts and at an early stage of larval life disappears alto- 

 gether. When the mouth is formed it is not provided with true 

 teeth, but has prominent lips which are marked with transverse 

 grooves and the ridges between these grooves are developed 

 into papilla; on the outer margin and more internally into 

 several transverse series of minute horny teeth formed by the 

 external horny layer of the epidermis. The jaws also within 

 the lips are covered by a horny beak composed of similar teeth 

 closely joined together. 



Four gill-clefts are formed and a third pair of external gills 

 grow out on the third pair of gill -arches ; the tail grows long 

 and develops a median vertical fin-membrane. The paired limbs 



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