xin PHYLUM CHORDATA 339 



by a special modification in the arrangement and mode of articula- 

 tion of the digits. In these remarkable arboreal Reptiles the three 

 innermost digits of the manus are joined together throughout 

 their length by a web of skin, and the two outer digits are 

 similarly united : the two sets of digits are so articulated that 

 they can be brought against one another with a grasping movement 

 analogous to the grasping movement of a Parrot's foot or of 

 the hand of Man. A similar arrangement prevails in the pes, the 

 only difference being that the two innermost and three outermost 

 digits are united. In some groups of Lacertilia, on the other hand, 

 such as the Blind- Worm (Anguis), limbs are entirely absent, or are 

 represented only by mere vestiges ; and numerous intermediate 

 gradations exist between these and forms, such as Lacerta, with 

 well-developed limbs. The limbless Lizards (Fig. 1)78) bear a very 





Fie. 978. Pygopus lepidopus, with scale-like vestiges of hind-limbs. (After Brehm.) 



close resemblance to the Snakes, not only in the absence of the 

 limbs, but also in the general form of the body and the mode of 

 locomotion. 



The body of a Snake is elongated, narrow and .cylindrical, 

 usually tapering towards the posterior end, sometimes with, more 

 usually without, a constriction behind the head. In the absence 

 of limbs the beginning of the short caudal region is only indicated 

 by the position of the cloacal opening. The fore-limbs are never 

 represented even by vestiges ; in some Pythons there are in- 

 conspicuous vestiges of hind-limbs in the form of small claw-like 

 processes. The mouth of the Snake is capable of being very 

 widely opened by the free articulation of the lower jaw, and it is 

 this mainly which distinguishes it from the snake-like Lizards. 

 But other, less conspicuous, points of distinction are the absence of 

 movable eyelids in the Snake, and also the want of a tympanum. - 



Sphenodon or Hatteria, the New Zealand Tuatara (Fig. 979), the 

 only living representative of the Rhynchocephalia, is a Lizard-like 



