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 vertebre 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 aérated, 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- 
bra join each other on a ball-and-socket plan (Fig. 246, 
4, c), and often number 4oo. 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 * 
