704 



PHYLUM CHORDATA : CLASS REPTILIA 



The mouth is expansible ; maxillce, palatines, pterygoids, 

 and quadrates are movable ; and the rami of the mandible 

 are connected only by elastic ligament. The teeth are fused to 

 the jaws ; there are no movable eyelids. Snakes have no 

 external ear openings nor drum, nor tympanic cavity, nor 

 Eustachian tube. The nostrils lie near the tip of the head. 



The bifid, mobile, retractile tongue is a specialised organ of 

 touch. In the mouth there is often a poison gland, which is a 

 specialised salivary gland. 



Fig. 413. — Anterior view of 

 Python's vertebra. 



N.SP., Neural spine ; ZS., zygo- 

 sphene (a projecting wedge) ; PR.Z., 

 pre-zygapophysis (smooth articular 

 surface seen from above) ; R., 

 articulation-surface for a rib; HY., 

 hypapophysis. 



er.z 



Fig. 414. — Posterior view of 

 Python's vertebra. 



ZA., Zygantrum, a double cavity 

 for the zygosphene ; PT.'C., post- 

 zygapophysis (smooth articular 

 surface seen from below) ; T.P., 

 transverse process ; C, centrum. 



There are many peculiarities in the skeleton. The numerous 

 vertebra are all procoelous. 



The brain has only ten nerves. 



The heart is three-chambered, the ventricular septum being 

 incomplete, as in all Reptiles except Crocodilians . 



There is a transverse cloacal aperture. In the males a 

 double saccular and spiny copulatory organ is eversible from 

 the cloaca. 



Snakes are widely distributed , but are most abundant in 

 the Tropics. 



