2 30 CHARACTERS OF VERTEBRATE ANIMALS 



In the absence of both sternum and pelvis, means of dis- 

 tinguishing between regions of the backbone largely fail, and 

 though the first two vertebrae are undoubtedly cervical, it is 

 best to speak of the rest as divided into two kinds only- 

 trunk vertebrae and tail vertebrae. Each of the former kind 

 bears a pair of simple curved ribs, the free ends of which are 

 firmly connected to a pair of the ventral epidermal shields. 



The most interesting features connected with the digestive 

 organs have reference to the tongue and teeth. The former 

 is slender and forked, and can be drawn back into a kind of 

 sheath. It is not a sting as is popularly believed, but probably 

 serves as an organ of touch, and may perhaps also play a part 

 in the " fascination " which a snake seems to exert over its 

 victims. The backwardly-curved, sharply-pointed teeth are in 

 most snakes arranged along the margins of the jaws and in two 

 rows along the palate. In poisonous forms the two front teeth 

 in the upper jaw are grooved or channelled for conducting poison 

 from special glands, and it is these "poison fangs" which are 

 capable of inflicting fatal wounds. Such a snake, therefore, 

 is deadly in virtue of its power of giving a poisoned bite. The 

 " sting " of a scorpion or wasp is an entirely different thing, and 

 the organ which administers it is situated at the hinder end of 

 the body, as most of us have practical cause to know in the case 

 of the latter animal. 



There is no occasion to dwell upon the circulatory organs, 

 for in essential respects these are constructed on the plan 

 described for the Sand Lizard (see p. 191). 



The breathing organs are distinguished by one or two 

 remarkable peculiarities. In the first place, the top of the 

 windpipe is drawn out into a tube, the end of which 

 protrudes from the mouth when prey is being swallowed, an 

 operation that would otherwise stop the breathing altogether. 

 The two lungs are very unequally developed, for whereas the 

 left one is reduced to a mere vestige, the other is correspond- 

 ingly large and extends far back in the body, the arrangement 

 being convenient in a body so elongated and narrow. The 

 hinder part of this right lung is thin-walled, and appears to 

 chiefly serve as a reservoir of air. 



The brain agrees in the main with the type described for 

 the Sand Lizard, and in regard to sense organs one peculiarity 



