VERTEBRATA. 97 
Iguana, although-of repulsive appearance, is highly esteemed as an 
article of food, and is the only Saurian reptile which is so used, except 
by savage tribes. 
Very different from all other reptiles in structure and general appear- 
ance is the last order, that of CHELONIANS, all of which are popularly 
known by the names of tortoise and turtle—the tortoises being those 
which live on land, and have their feet adapted chiefly for walking ; 
the turtles, those which live in water, and whose limbs are mere 
paddles for swimming. The Chelonians are all inhabitants of warm 
countries. Their numbers in some places are astonishingly great. 
The species both of tortoises and turtles are very numerous, and 
some of them are highly esteemed for food. The useful substance 
called tortoise-shell is the hard covering of animals of this order. 
This covering is really bony, and therefore differs from the outer 
covering of all other vertebrate animals, being formed from the ribs, 
the vertebra, and the breast-bone expanded into plates, and firmly jointed 
into each other, so as to acquire great solidity, particularly in tortoises, 
and give most perfect protection to creatures which have no other means 
of defence. Tortoises feed exclusively on vegetable food, but some of the 
aquatic Chelonians pursue and prey upon other aquatic animals. The 
jaws of Chelonians are not furnished with teeth, but are hard, sharp, and 
horny ; they act in a manner somewhat resembling the mandibles of 
birds. All the Chelonians are oviparous. They lay a great number of 
eggs, which are covered with a calcareous shell like those of birds, whereas 
those of other reptiles have only a parchment-like covering. The eggs of 
turtles are a favourite luxury in the countries in which they can be 
obtained. 
Birds.—Birds are the highest class of oviparous animals, and the 
only class having warm blood. They exhibit a great similarity in their 
general structure, and differ very widely in appearance and characters 
from every other class of animals. To this class belong all the vertebrate 
animals which are capable of true flight, except bats. The fore pair of 
limbs in birds serve them only as wings, never in any degree as arms or 
legs ; those few birds in which they are too small to raise the body in the 
air, generally employ them to aid their swift running upon land, as the 
ostrich, or for swimming wnder water, as penguins, which in this manner 
pursue fish. The body of birds is covered with feathers, and this is one 
character in which all birds agree, and in which they differ from all other 
animals. The jaws of birds are much elongated, so as to form the bill, the 
organ chiefly used in seizing food. The bones of the wing are essentially 
the same with those of the arms of man, but very much modified to 
accommodate them to their different use. The bones which represent 
the fingers are much condensed and partially obliterated. The surface 
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