TRUE REPTILES. 205 



directly from the egg. They have many features in com- 

 mon with the birds, as true nostrils and a long windpipe. 

 About three thousand living reptiles are known. 



The Skeleton. The skeleton varies much in different 

 orders. The vertebrae in many of the snakes consist of 

 three hundred different bones working on a ball-and-socket 

 plan. The limbs in the various forms are adapted for walk- 

 ing or swimming, as the case may be. 



Digestion. With the exception of the turtles, that have 

 a horny mandible, the reptiles all possess sharp teeth, ar- 

 ranged either in rows or separate cavities, that are adapted 

 for crushing, cutting, or holding prey. The stomach of 

 snakes is but little removed from an ordinary intestine. 

 In the crocodiles it resembles the gizzard of birds. 



Circulation. In the crocodile the heart is four-cham- 

 bered as in the birds, while in other forms there is, besides 

 the two auricles that are always present, but one ventricle. 

 The blood is imperfectly aerated, consequently the reptiles 

 are cold-blooded. 



Respiration. The reptiles breathe by lungs alone ; and 

 here we first find a true nostril, as in the birds and higher 

 forms. In the snakes the lung is single. 



Development. The reptiles are oviparous, the egg-shell 

 generally being soft and crisp. Some are ovoviviparous, 

 or the young are developed before the egg is laid. 



Order I. Snakes (Ophidia). General Characteristics. 

 The snakes are distinguished by their long, cylindrical, 

 footless bodies. The bones of the backbone or verte- 

 brae join each other on a ball-and-socket plan (Fig. 246, 

 6, c), and often number 400. The bones of the lower jaw 

 are merely connected by ligaments, ^, allowing great ex- 

 tensibility. The teeth are not set in sockets, and point 

 backward, being only used in holding prey. The tongue 

 is extensile, and held within a sheath. The eyes are with- 

 out movable lids, hence the staring expression of all snakes. 

 The gliding motion is effected by the successive advancing 



