682 PHYLUM CHORDATA : CLASS AMPHIBIA 



Although the life of Amphibians seems to have on an average a low 

 potential, even the most sluggish wake up in connection with repro- 

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

 vSome of their parental habits seem like strange experiments. 



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

 fertilised internally and placed by the everted cloaca of the female upon 

 the back, the male apparently helping in the process. The skin 

 becomes much changed — doubtless in response to the strange irritation 

 — and each fertilised ovum sinks into a little pocket, which is closed by 

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

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

 miniature adults. In Nototrema the female has a dorsal pouch of skin 

 opening posteriorly, and within this tadpoles are hatched. In Rhino- 

 derma 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 approaching completion, then plunges 

 into a pool, where he is freed from his living burden. 



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

 oviduct ; in most Urodela fertilisation is internal, sometimes by 

 approximation of cloacae, sometimes by means of complex spermato- 

 phores 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. 



There are about 900 living species, most of them tailless. Almost 

 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 Piscine 

 stock related to the Dipnoi and perhaps also to the Crossopterygians. 

 The Stegocephali were the first pentadactyl animals (Lower Carboni- 

 ferous). 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 renaain cartilaginous ; the quadrate ossification is 

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

 of the mandible, thus affording eas^^ transition to the mammalian con- 

 dition of these parts. But Mammals are, on the whole, more directly 

 related to Reptiles. 



There are some remarkable affinities between the Stegocephali and 

 some of the extinct Reptiles, such as the Anomodonts, which in their 

 turn have affinities with Mammals, 



