LIFE OF AMPHIBIANS. 559 



pressicauda, the fertilisation must take place internally, for the eggs 

 are hatched within the mother. 



The eggs of the frog are laid in masses, each being surrounded by a 

 globe of jelly ; those of the toad are laid in long strings ; those of newts 

 are fixed singly to water-plants ; those of some tree-toads, such as 

 Hylodes, are laid on or under leaves in moist places. 



In Salamandra atra, Pipa americana, Hylodes, and Cacilia com- 

 pressicauda, the young are hatched as miniature adults ; and marked 

 metamorphosis can hardly be said to occur in any Urodela. The larval 

 stages of Amphibians afford clear illustration of the plasticity of young 

 animals under environmental stimulus. Thus the larvte of Salamandra 

 maculosa become lighter or darker as the water is warmer or colder, 

 and the tadpoles of frogs and young salamanders becomes lighter in 

 darkness and darker in light, though the opposite is true of adult frogs. 



There are about 900 living species of Amphibia, most of them tail- 

 less. All are averse to salt water, hence their absence from almost all 

 oceanic islands. The Anura are well-nigh cosmopolitan ; the Urodela 

 are almost limited to the temperate parts of the northern hemisphere. 



History. — It is likely that Amphibians were derived from a stock 

 from which the Dipnoi and perhaps also the modern Elasmobranchs 

 sprang. The Stegocephali or Labyrinthodontia really include two or 

 more distinct orders. Of living forms, the Gymnophiona are more old- 

 fashioned than the others. The modern types gradually appear in 

 Tertiary times. Some of the extinct forms were gigantic. 



Huxley has emphasised the following affinities between Amphibians 

 and Mammals : — The Amphibia, like Mammals, have two condyles on 

 the skull ; the pectoral girdle of Mammals is as much amphibian as it 

 is sauropsidian ; the mammalian carpus is directly reducible to that of 

 Amphibians. In Amphibians only does the articular element of the 

 mandibular arch remain cartilaginous ; the quadrate ossification is 

 small, and the squamosal extends down over it to the osseous elements 

 of the mandible, thus affording easy transition to the mammalian con- 

 dition of these parts. 



There are many remarkable affinities between the Labyrinthodont 

 Amphibians and a class of extinct Reptiles known as Anomodontia, 

 and as the latter have also many affinities with Mammals, it is possible 

 that both Mammals and Anomodonts diverged from an Amphibian 

 stock. The strange extinct Eotetrapoda of Credner seem to unite the 

 Stegocephali to the Rhychocephalia, a class of Reptiles now repre- 

 sented by the New Zealand ' ' lizard " Sphenodon. 



