Chap. 35 REPTILES FIRST LAND VERTEBRATES 721 



VERTEBRAL COLUMH';^ 



-I ANUS 



■CLAW 



FEMUR -" 

 CLAW- 



Fig. 35.9. Remnants of hind legs and pelvic girdle of a python, indicating that 

 the ancestors of snakes once traveled on legs. A, ventral external region where 

 claws extend out between the scales. B, skeleton in the same region. The hip girdle 

 is represented only by a slender ilium, embedded in the flesh on each side. The 

 limbs are vigorously moved and the claws are capable of inflicting deep cuts. 

 Pythons and boas are constricting snakes, some of them 30 feet long, with jaws 

 capable of opening widely enough to take in a sheep. (Courtesy, Rand: The 

 Chordates. Philadelphia, The Blakiston Co., 1950.) 



ovaries or testes are present, but one lung is in front of the other, one kidney 

 in front of its mate and so on. It is a tandem series. 



Ribs play various parts in the activities of snakes. They stiffen the spreading 

 hoods of cobras (Fig. 35.8), and urged by the muscles that control them, they 

 squeeze the still-living animal that the snake has swallowed. Contractions of 

 muscles in the body wall, contractions of rib muscles, and of those that lift 

 the ventral scales all take part in locomotion. This is either a glide straight 

 forward or a curving slither alternately from side to side like a swimming eel. 

 A snake seems to slide without effort. It is not surprising that Solomon found 

 "the way of a serpent upon a rock" one of the things that baffled his mind. 



Snakes are pure carnivores. Common garter snakes prey upon insects and 

 other animals up to the size of frogs. Rattlesnakes do likewise and can swallow 

 small rabbits. The bones of the lower jaw have elastic joints allowing the 

 necessary great stretch. The snake hooks its teeth into the victim, first on one 

 side, then the other gradually pulling its mouth over the rabbit. Teeth, espe- 

 cially the poison fangs, are often broken but partly developed ones behind 

 them immediately take their places (Fig. 35.10). A snake travels by its tongue 

 as a dog travels by its nose. Slipping leisurely along with its mouth tightly 

 closed it explores every object with this ominous, flashing, black and red, 

 but entirely harmless organ. It is lodged in a sheath in the floor of the mouth 

 and extended through the small opening formed by a notch in each jaw. 



Poisonous Snakes. Of some 2500 living species of snakes about 600 are 

 more or less poisonous. The venom is secreted by modified salivary glands in 

 the upper jaw and injected into the wound by the fangs which are grooved or 

 tubular teeth. The venom contains poisonous proteins whose proportions vary 



