446 AMPHIBIA. 



tion. The males often differ from their mates in size and colour. Some 

 of their parental habits seem like strange experiments. 



Thus in the Surinam toad {Pipa americana), the large eggs are placed 

 by the male on the back of the female and fertilised there. The skin 

 becomes much changed — doubtless in response to the strange irritation 

 —and each fertiUsed ovum sinks into a httle pocket, which is closed by 

 a gelatinous lid. In these pockets the embryos develop, perhaps absorb- 

 ing some nutritive material from the skin. They are hatched as 

 miniature adults. In Kototrema and Opisthodelphis, the female has a 

 dorsal pouch of skin opening posteriorly, and within this tadpoles are 

 hatched. In Rhinoderma darwinii, 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 responsibility of hatching 

 the eggs. 



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

 oviduct ; in the newt the male deposits a spermatophore in the water 

 close to the female ; in Salamandra atra, S. maculosa, and Cacilia 

 comfressicauda the fertilisation must take place internally, for the young 

 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 Ccecilia com- 

 pressicauda, the young are hatched as miniature adults ; in the others 

 there is more or less metamorphosis after birth. 



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 limited to the temperate parts of the northern hemisphere. 



The Frog {Rand) as a type of Amphibians. 



The common British frog (Rana temporarid) and the 

 Continental species (R. esculentd) often imported agree in 

 all essential features. A black patch on the side of the 

 head distinguishes R. ttmporaria ; the males of R. escuknta 

 have croaking sacs, and many other unimportant differences 

 are noticeable. 



Though aquatic in youth, frogs often live in dry places, 

 hiding in great drought, reappearing when rain returns. 

 Every one knows how they sit with humped back, how they 



