524 AMPHIBIA [CH. 



The ventral wall of the cloaca, however, is produced outwards 

 into a great thin-walled sac, the allantoic bladder, in which when 

 the cloaca is closed the urine accumulates. This organ acquires 

 immense importance in the development of the higher animals and 

 is found in no fish. 



In the larva, which is to all intents and purposes a fish, there are 

 present those peculiar sense-organs called mucous canals, supplied 

 by the 5th and 10th nerves, but these are usually lost in the adult. 



Living Amphibia are divided into three well-marked orders, 

 viz. the URODELA, the ANURA and the APODA. The 

 URODELA (Gr. ovpd, tail ; S-fjXos, conspicuous) have 

 long cylindrical bodies and long flattened tails. The limbs are 

 short and comparatively feeble, barely strong enough to lift the 

 belly from the ground. Both pairs of limbs are about equal in 

 size. The ANURA (Gr. dv-, 110 ; ovpd, tail) have much broader and 

 shorter bodies; the tail is totally lost and the hind limbs are 

 powerfully developed and adapted for jumping. The APODA (Gr. 

 a-j no, : TTOVS, TroSo's, a foot) have lost both pairs of limbs and 

 their cylindrical bodies give them a worm-like appearance; their 

 habits heighten the resemblance since they burrow in moist earth. 

 They have embedded in the skin small bony plates, relics of the 

 scales which their fish-like ancestors once possessed. The tail has 

 in these animals almost disappeared. 



In the Carboniferous rocks the remains of a large number of 

 Amphibia have been found which have been called STEGOCEPHALA 

 (Gr. ore/OS, a roof ; K<f>aX->j, the head) from the circumstance that 

 the head is covered with a compact mosaic of membrane-bones 

 extending from the mid-dorsal line of the cranium outwards to 

 the lips. Similar small bones or scales are found on the ventral 

 surface. These features show resemblances to what is found in 

 Dipnoan fish from which Amphibia are probably descended, and 

 the small scales of the Apoda seem to be the last remnants of 

 this armature. Stegocephala include both long and short tailed 

 forms, and while some of their descendants the Labyrinthodonta 

 became highly specialised in the structure of their teeth and 

 died out in the next geological period, others, in all probability, 

 gave rise to modern Amphibia. 



Order I. Urodela. 



Returning to the Urodela, which are the most primitive of 

 modern Amphibia, we find that in Great Britain they are represented 



