410 REPTILES xv. 9- 



heloderms from North and Central America are also playtnotids and are 

 of special interest as the only known poisonous lizards. Another allied 

 form is the rare earless monitor, Lanthanotus, from Borneo, which 

 seems to be a survivor of the primitive platynotid stock. 



Many lizards are able to change colour, the chameleons, Anolis 

 (often called 'American chameleons'), and certain agamids being the 

 most notable for this. The colour may change with the environment, 

 serving the obvious purpose of concealment. Special colour patterns 



Fig. 235. Diagram of the nervous control of the melanophores in the chameleon 

 (above) and minnow (below). 



c, spinal cord; f, pathway of pigmentomotor fibres (the synapse in the sympathetic ganglia 



is omitted); M, melanophore; n, spinal nerve; p, pigmentomotor centre; s, sympathetic. 



(From Sand, minnow after v. Frisch.) 



are displayed in courtship or threat, and colour change may also 

 occur in response to temperature and other environmental changes. 

 The physiological mechanism of colour control varies in different 

 reptiles. In Anolis there are probably no nerves to the melanophores, 

 which are controlled by hormones produced by the posterior pituitary 

 and possibly other glands. In chameleons, however, the melanophores 

 are controlled partly or entirely by the autonomic nervous system 

 (Fig. 235). 



Many lizards are able to break off their tails when threatened or 

 seized by a predator; this ability, known as autotomy, is due to the 

 presence of special planes of weakness through the bodies of the caudal 

 vertebrae. Such fracture planes are also found in Sphenodon, but not 

 in snakes. After autotomy the tail regenerates, but the new member 

 is not a replica of the normal. The vertebrae, for instance, are not 

 regenerated, their place being taken by an unsegmented tube of car- 

 tilage. It has been shown experimentally that in Anolis regeneration 

 will not occur if the motor-nerves are prevented from growing back 



