174 AMPHIBIA. 



internal chamber as in Fishes. In some cases the external 

 branchiae only are present, and they are, in any case, the gills 

 which are retained in those forms in which the branchiae are 

 permanent {Perennibranchiata). In the tailed Amphibians 

 (Urodela) and in the Frogs and Toads {Anoura) two sets of 

 gills are developed — an external set, which is very soon lost, 

 and an internal set, which is retained for a longer or shorter 

 period. As maturity is approached, true lungs adapted for 

 breathing air are developed. The development, however, of 

 the lungs varies with the completeness with which aerial 

 respiration has to be accomplished ; being highest in those 

 forms which lose their gills when grown up [Caditcibran- 

 cliiata), and lowest in those in which the branchise are re- 

 tained throughout life {Perennibranchiata). 



The class Amfphibia is divided into the four orders of the 

 OpMomorpha, Urodela, Anonra, and Lahyrintliodontia. The 

 first of these includes only the serpentiform animals known 

 as Cmciliai, and not having any certain fossil representatives, 

 may be altogether passed over here. The order Urodela 

 comprises the so-called " tailed " Amphibians of the present 

 day, such as the Newts and Salamanders. The earliest 

 traces of this order in past time, with some doubtful excep- 

 tions, occur in the Tertiary deposits. The order Anoura 

 includes the so-called " tail-less " Amphibians, such as the 

 Frogs and Toads, and is not known to have existed in 

 periods anterior to the Tertiary. Lastly, the order Lahj- 

 rintliodontia is entirely extinct, and is known to have ex- 

 isted mainly during the Carboniferous, Permian, and Triassic 

 periods. 



Order I. Urodela ( = IcMkyomovpha., Owen ; Saurohatra- 

 cliia). — This order is commonly spoken of collectively as that 

 of the " Tailed " Ampliibians, from the fact that the larval tail 

 is always retained in the adult. The Urodela are charac- 

 terised by having the skin naked and almost invariably 

 destitute of any exoskeleton. The body (fig. 537) is elon- 

 gated posteriorly to form a compressed or cylindrical tail, 

 which is permanently retained throughout life. The dorsal 

 vertebrffi are biconcave {amphicmlons)^ or concave behind and 

 convex in front {o2yistJwca;lo2is), and they have short ribs 



