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EVOLUTION OF ANIMALS 



Part V 



Non poisonous snake without pit 



Poisonous snake with pit. 



Section through the interlocked 

 scales of a rattle 



B 



Fig. 35.3. Scales of snake. A, head of a non-poisonous pilot snake. B, side of 

 the body of a snake with smooth scales; the anal region and tail showing the 

 large ventral scales. C, head of poisonous copperhead snake. The pit between the 

 eye and opening of the nostril is characteristic of poisonous snakes. D, section 

 through the tip of the tail of a rattlesnake showing the loosely interlocked scales 

 which are rattled. (A, B, and C, courtesy, Surface: Serpents of Pennsylvania. 

 Harrisburg, Penna. State Dept. of Agric, 1906. D, courtesy, Weichert: Anatomy 

 of the Chordates. New York, McGraw-Hill Book Co., 1951.) 



Characteristics of Reptiles. The outer layer of skin is dry and horny, usually 

 with small scales in lizards and snakes and very large ones (scutes) in turtles 

 and crocodilians. The ancient reptiles and the modern lizards, alligators, and 

 others have two pairs of limbs, typically with five toes that end in horny claws 

 (Fig. 35.5). Their bodies are low slung, adapted to running close to the 

 ground, to climbing in many lizards, and to crawling in alligators. Limbs are 

 reduced or absent in some lizards and in all snakes. 



The reptilian skeleton is relatively heavy, and contains more calcium than 

 that of fishes or amphibians. Except in turtles and snakes, the ribs are moved 

 during breathing much as they are in birds and mammals. Reptiles have a 

 distinct neck region, and were the first vertebrates that could turn their heads 

 sidewise. Even the sea turtles breathe chiefly by means of lungs. Eyes and other 

 sense organs are adapted to life on land, always protected from exposure to 

 air. The temperature of the body, always the expression of its metabolism, is 

 low and varies with that of its surroundings. This has limited reptiles to long 

 hibernations or to life in subtropical regions. For example, Louisiana has 

 over 70 species of reptiles; northern Alberta has one, a garter snake. 



Fertilization is internal, a protection of the sex cells from drying. The eggs 

 are large with abundant yolk, and in leathery or limy shells. The majority of 

 reptiles are oviparous, and their eggs are incubated and hatch outside the body. 

 Some lizards and snakes are ovoviviparous; the eggs, fertilized and later 

 supplied with shells, are incubated and hatched within the oviducts from which 

 the shells are later expelled. In essentials, this process is intermediate between 



