398 



METAZOAN PHYLA 



The pit vipers (Fig. 288) have long, curved fangs near the anterior 

 end of the upper jaw. When the mouth is closed they he against its 

 upper wall, but when it is opened widely they are raised and stand nearly 

 at right angles to this surface. In this position when the snake strikes, 

 they are driven straight forward into the body of the animal struck; 

 at the same moment poison is injected into the wound from the poison 



Cranium 



Palatine 



Fig. 288. — Skull of rattlesnake, Crotalus confluentes Say. From a young specimen from 

 Montana. A, entire skull, seen from the side and below. B, lateral view, with jaws of 

 only one side shown. The erection of the fang is caused by the thrust of the ectopterygoid 

 against the movable maxilla. When the mouth is closed the ventral end of the quadrate, 

 to which the lower jaw is articulated, is carried backward, the palatine and pterygoids are 

 brought up toward the floor of the cranium, and the fangs lie against the roof of the mouth. 

 But when the mouth is opened the articulation of the quadrate and lower jaw is brought 

 forward, causing the palatine and pterygoids also to be carried downward and producing a 

 forward movement of the ectopterygoid, which in turn erects the fang. In C and D is 

 shown a mechanism which would work in the same fashion. 



glands at the bases of the fangs. Snakes use their poison fangs both in 

 securing prey and in defending themselves from enemies. When fangs 

 are lost they are replaced by other fangs which lie concealed behind the 

 functional ones and come up one at a time to take the place of the lost 

 fangs. 



420. Rhynchocephalia. — The only representative of this order is 

 the tuatara of New Zealand, Sphenodon punctatum (Gray). It was 

 formerly found throughout New Zealand but is now restricted to some 



