434 EVOLUTIONARY MORPHOLOGY 



right angles to the body, and as the body performed the same 

 undulatory movements by means of the myotomes as does a 

 fish when swimming, the limbs were moved forwards and 

 backwards. In other words, the limbs were used as oars to 

 row the animal along on land, and the same muscles and 

 nervous connexions came into play as in the aquatic ancestor. 

 In the very earliest Amphibia, the sacrum was absent {e.g. 

 Eogyrinus). In the others it was present, and by anchoring 

 the pelvic girdle on to the vertebral column, it strengthened 

 the hind limbs. 



One consequence must be mentioned of the possession of 

 an autostylic method of suspension of the jaws, and of the 

 abolition of branchial respiration and the closure of the visceral 

 clefts, in particular the spiracle. The hyomandibula being 

 no longer required to suspend the quadrate from the auditory 

 capsule, its function became converted into that of conveying 

 vibrations from the skin covering the spiracular cleft to the 

 auditory capsule. In this way the hyomandibula became the 

 columella auris ; the covering of the spiracular cleft became 

 the tympanic membrane, and the cavity of the spiracular cleft 

 became the middle-ear and Eustachian tube ; all quite simply 

 and without involving any great rearrangement. So the ear 

 became an organ for the delicate appreciation of sound as well 

 as balance. 



The early amphibia had a covering of dermal bones more or 

 less all over the body. The vertebral column of the earliest 

 forms or Embolomeri is remarkable in that each vertebra 

 possessed two centra. There was an anterior hypocentrum 

 and a posterior pleurocentrum. The later amphibia preserved 

 the hypocentrum at the expense of the pleurocentrum, which 

 disappeared. It will be seen that in the reptiles the opposite 

 occurred. 



Collectively, the early amphibia are known as the Stego- 

 cephalia or Labyrinthodonts, the former term referring to the 

 complete bony covering of the skull. They flourished in the 

 Carboniferous, and persisted until the Triassic period, when 

 they were extinguished by the competition of their more 

 successful descendants the reptiles, leaving only the frogs 



