XI AMPHIBIA 417 



girdle. In the Urodeles the coracoid portions of the girdle are broad 

 flat plates of cartilage which overlap one another. In most of the Anura 

 the two halves of the shoulder girdle meet ventrally without overlapping 

 and the line of their junction is prolonged forwards and backwards as 

 a skeletal structure analogous with the sternum or breast-bone of higher 

 vertebrates. 



In the pelvic girdle the increased firmness is attained in different 

 fashion through the tip of the iliac bone being attached to the tip of the 

 transverse process of one or sometimes two sacral vertebrae. In the 

 Anura^ in correlation with their specialization for leaping and in order to 

 bring the thrust of the hind-legs well forwards, the attachment of the 

 pelvic girdle is relatively far forward, there being commonly only eight 

 pre-sacral vertebrae and in Hymenochirus only five. In these Anura 

 the portion of vertebral column behind the sacrum has degenerated and 

 become converted into an unjointed rod of bone. 



The brain of the amphibian with its large hemispheres and feebly 

 developed cerebellum resembles that of the lung-fish. In the Amphibia 

 we see for the first time a conspicuous sympathetic nerve-trunk — a 

 longitudinal trunk lying just external to the dorsal aorta and aortic 

 roots, bearing at intervals masses of ganglion cells (sympathetic ganglia) 

 each connected with a spinal nerve by a little bridge of nerve-fibres 

 (" ramus communicans "). The sympathetic nervous system is con- 

 cerned with the control of the muscular coat of blood-vessels and ali- 

 mentary canal. Thus in the case of the blood-vessels the muscular coat 

 is kept in a state of tonic contraction so that the vessels are of medium 

 calibre. By intensifying the control the calibre is reduced, by slacking 

 it the calibre is increased. In this way the blood-supply to the various 

 parts of the body is regulated. 



A remarkable peculiarity exists in the otocyst of the Anura, the wall 

 of the endolymphatic duct sprouting out and fusing with its fellow to 

 form an irregular thin-walled sac full of otolithic particles and overlying 

 the fourth ventricle. This sac continues to grow, spreading ventrally 

 and also tailwards along the dorsal side of the spinal cord. Special 

 outgrowths sprout out along the course of the spinal nerves and form 

 the " calcareous bodies," conspicuous chalky-white sacs which surround 

 the spinal ganglia, the colour being due to the otolithic particles in their 

 interior. 



The Amphibia are of special interest from the extent to which they 

 have been able to emancipate themselves from the ancestral watery 

 medium and become terrestrial. They have never been able to do 

 so completely however. A moist skin is essential to their hfe and 



2 E 



