LOCOMOTION OF LIMBED REPTILES. 



263 



1G2 



Toads, with semipalmate feet, have an awkward, but not always 

 slow, progression on land by alternate movements of the limbs. 

 Some species are enabled, by peculiar tubercles or projections 

 from the palm or sole, to clamber up old walls. 



But the most remarkable climbers in the reptilian class are 

 certain lizards, especially those called ' Geckos.' Each foot has 

 five toes, which are spread wide apart, expanded at the ends, and 

 terminated by a slender sharp claw. The flattened under surface 

 of the toe-pad, fig. 162, is traversed by a series of 

 transverse folds, with deep interspaces ; the 

 margins of the folds, when applied to a smooth 

 surface, adhere thereto by atmospheric pressure, 

 through the vacuum caused by muscular erec- 

 tion of the folds, with concomitant expansion of 

 the interspaces ; thus the animal, alternately 

 applying and releasing its suckers, climbs a 

 vertical wall or plate of glass, or proceeds along 

 a ceilino- with its back downward. 



For climbing trees the adjustment of the toes 

 in opposition to each other, in equal or sub- 

 equal groups, as already described in the Cha- 

 meleon, pp. 175, 193, figs. 119, 123, is an 

 effective modification. In this reptile the limbs 

 are short and strong, and a prehensile tail is 

 added to the scansorial feet. 



Ordinary lizards, by the great length of the trunk in proportion 

 to its breadth, and by the outward extension of the humerus and 

 femur, are under unfavourable mechanical conditions for rapid 

 course upon land. Yet such is the energy of the muscular con- 

 tractions in some species, under the stimulus of solar heat, that 

 they are deservedly called f agile,' and ' dart ' out of view in the 

 first rush from a pursuer. They have not, however, the power of 

 maintaining the exertion, and are soon overtaken, if they happen 

 to be far from their retreat. The swiftest runners, e. g. the Tachy- 

 dromi, have the fore and hind limbs least differing in length, and 

 consequently the vertebral column most parallel with the plane of 

 motion. 



In the Crocodiles the fore limbs are shorter than the hind ones, 

 in which the foot is longer and more expanded, so as to present a 

 larger surface for striking the water in swimming. But the chief 

 natatory organ in these large amphibious reptiles is the long, 

 compressed tail. In the act of swimming, the fore limbs are 

 applied flat to the sides of the body, and the hind ones chiefly 



Toe of Gecko, magn. 

 coin. 



