LIFE OF AMPHIBIANS. 589 



hatched. In Rhinoderma danviuii the male carries the ova in his 

 capacious croaking-sacs. In the case of the obstetric toad (Alytes 

 obstetricans], not uncommon in some parts of the Continent, the male 

 carries the strings of ova on his back and about his hind-legs, buries 

 himself in damp earth until the development of the embryos is ap- 

 proaching completion, then plunges into a pool, where he is freed 

 from his living burden. Thus among Amphibians, as among Fishes, 

 the males sometimes take upon themselves the task of hatching the 

 eggs. 



In the Anura the ova are fertilised by the male as they leave the 

 oviduct ; in most Urodela fertilisation is internal, sometimes by approxi- 

 mation of cloacae, sometimes by means of complex spermatophores which 

 the male deposits in the water close to the female. 



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. 



The larval stages of Amphibians afford clear illustration of the 

 plasticity of young animals under environmental stimulus. Thus the 

 larvne of Salauiandra niaculosa become lighter or darker as the water is 

 warmer or colder, and the tadpoles of frogs and young salamanders 

 become 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 were the first pentadactyl animals (Lower 

 Carboniferous). 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 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. But Mammals are more nearly related to 

 Reptiles. 



There are some 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 Rhynchocephalia, a class of Reptiles now repre- 

 sented by the New Zealand "lizard" Sphenodon, 



