HIGH SCHOOL ZOOLOGY. Ill 



17. From such footless lizards to the true snakes, Ophidia, the 

 transition is sufficiently easy. Among the latter, indeed, are 

 some forms which retain rudimentary hind limbs ; such ai'e the 

 Pythons, Boa Constrictors and Anacondas of the Old and New 

 World ; again there are other smaller forms like the blind snake 

 (Typhlops), which show the burrowing habit and the external 

 form of the Ann)hisb;ena, but lack the peculiar arrangement of 

 the jaws which we see in the ty[)ical snake. All our Ophidia, 

 however, belong to two families which exhibit considerable dif- 

 ference from the structure of any lizard. Not only are the fore 

 and hind limbs absent, but there is no trace of the girdles sup- 

 porting them, nor of a sternum. Locomotion, being effected 

 by the ends of the ribs and by shields of the ventral surface 

 whose hinder edges are free, presents a great contrast to the 

 clumsier movements of the footless lizards. In the latter, certain 

 of the organs are aftected by the lengtii of the body, the ten- 

 dency being for paired organs like the lungs and oviducts to 

 become unequal in size, one of them assuming the function of 

 both, but this tendency is carried further in the snake, so that 

 one lung or one oviduct may alone be present. 



18. The absence of the pectoral and pelvic girdles makes it 

 impossil)le to recognise any but the trunk and qaudal regions of 

 the vertebral column. A neck may be present in the form of a 

 constricted part of the trunk behind the head, as in the Rattle- 

 snake, but its vertebrpe do not differ from those of the region 

 behind it. So heterogeneous an order is that of the lizards, that 

 we meet with the greatest variety in the epidermal coverings of 

 the body, but the Ophidia constitute just as homogeneous a 

 group on the other hand ; nevertheless, in spite of the ai^parent 

 similarity of the scales and shields, slight differences in the form 

 and arrangement of tliese are used by systematists in the 

 diagnoses of the species. (Fig. 82). The epidermis is cast off 

 several times a year in the form of a slough, the first moult 

 taking place immediately after the escape from winter quarters. 



