326 COMPARATIVE ZOOLOGY OF CHORDATES 



the tail disappears altogether in the adult terrestrial form ; in 

 the Urodeles (newts) it persists as a more or less tubular 

 structure. In the Gymnophiona there is scarcely any tail at 

 all, for the anus is almost at the hind extremity of the animal. 

 In land-animals, the tail ceases to have the function which 

 it exercised in the water, and it is often consequently much 

 reduced. Instead of being a posterior prolongation of the 

 body, it has the appearance of being merely an appendage, 

 and it is of use to the animal in the maintenance of its balance, 

 as a covering for the anus and genitalia, and in some cases as 

 a fly- whisk. 



Lizards have an interesting modification in that the 

 vertebrae of the tail are cleft transversely, and it is at these 

 points that the tail can be detached from the rest of the body. 

 This faculty (autotomy) is of service to the animal in enabling 

 it to escape from its enemies. 



The primitive birds had long tails, with separate vertebrae, 

 as is shown by Archaeopteryx. In living birds the caudal 

 vertebrae are fused together to form the pygostyle, and the tail 

 itself is much reduced. The so-called tail of birds consists 

 of the tail-feathers. 



In some arboreal animals, such as the chamaeleon and the 

 American monkeys, the tail is prehensile and capable of grasping 

 objects. 



It is common to find that in those vertebrates which 

 have returned to the water, the tail is well developed and 

 expanded into fins. While superficially not unlike the tails 

 and caudal fins of fish, they show in their structure fundamental 

 differences. So in Ichthyosaurus, the vertebral column passes 

 back into the ventral lobe of the fin ; in the whales the two 

 lobes of the caudal fin are not dorsal and ventral but right and 

 left, for the tail is expanded horizontally. 



In the apes and man the external tail has disappeared 

 altogether. 



