312 ZOOLOGY sect. 



before the metamorphosis was discovered its connection with 

 Amblystoma was not suspected, and it was placed in a distinct 

 sreuus, Sircdon, among the Pcrennibranchiata. 



Segriicntalion of the egj:^ in the Anuva and Urodela is always com- 

 plete but unequal. In Pipa and Alytes there is a large quantity 

 of food -yolk, and the developing embryo lies on the surface of 

 a large yolk-sac. In the Gymnophiona the eggs, Avhich are 

 singularly like those of a Bird, are of large size and segmentation 

 is partial, the formation of segments at the pole of the Qgg opposite 

 that at which the formation of the embryo begins only taking- 

 place at the stage of gastrulation : the embryo is coiled over the 

 surface of the yolk as in the Trout. 



Distribution. — The Urodela are almost exclusively Pahearctic 

 and Nearctic forms, occurring in North America, Europe, Asia, and 

 North Africa : a few species extend southwards into the Neotropical 

 and Oriental regions. The Gymnophiona, on the other hand, are 

 mainly southern, occurring in the Neotropical, Ethiopian, and 

 Oriental regions, but are absent in Australasia and the Pacific 

 Islands. The Anura are almost universally distributed, and are 

 abundant in all the greater zoo-geographical regions : they are, 

 however, represented in New Zealand only by a single species 

 {Liopchna hochdctteri), very locally distributed, and are absent in 

 most Oceanic islands, a fact due to the fatal effects of salt 

 water upon the eggs and embryos of Amphibia as well as upon the 

 adults. 



Remains of Stegocephala arc found in considerable abundance 

 from the Carboniferous to the Trias, and one genus extends into 

 the Lower Jurassic, after which period the order apparently became 

 extinct. The Urodela and Anura are not known until the Eocene, 

 and no fossil remains of Gymnophiona have been found. 



Mutual Relationships. — The perennibranchiate Urodela are 

 undoubtedly the lowest of existing Amphibia ; they lead up, through 

 such forms as Amphiuma, with persistent gill-slits but deciduous 

 gills, to the Land Salamanders, in which a purely terrestrial form is 

 assumed. The Stegocephala exhibit a parallel series of modifications, 

 some of them being perennibranchiate, others caducibranchiate. 

 Their skull is more complex than that of the Urodela, but their 

 vertebral column never reaches the same degree of specialisation as 

 that of the Land Salamanders, and in some cases shows a lower 

 grade of organisation than in any existing Amphibia. Both in 

 their skeleton and in tlie distribution of their lateral sense-organs 

 they show some affinity with the Crossopterygii. The Anura 

 are a very specialised group : their development indicates their 

 derivation from branchiate tailed forms, but there is no palseonto- 

 logical eviduuce on this jjoint. 



