386 



METAZOAN PHYLA 



Among the snakes called vipers are the pit vipers, represented in 

 the New World by the many species of rattlesnakes, the water moccasin, 

 and the copperhead. In all of these the young are brought forth alive. 

 The water moccasin, found in southern United States, is one of the 

 most poisonous of all snakes. The copperhead, found farther north, 

 is also very venomous. Rattlesnakes possess a series of hollow epidermal 



Cranium 



Palatine 



Fig. 263. — Skull of rattlesnake, Crotalns 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. 



buttons linked together and forming a rattle attached to the tip of the 

 tail, which is itself flattened, hardened, and constricted to receive the 

 anterior margin of the basal rattle. A new button is added to the series 

 of rattles every time the skin is shed. Since this may occur several 

 times a year, and since rattles may be lost, the number possessed by an 

 individual is no precise indication of the age of the snake. 



The pit vipers (Fig. 263) have long, curved fangs near the anterior 

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



