INTERNAL ORGANS 
85 
it has a pair of kidneys that remove the wastes from the blood 
in the form of liquids; while the skin and lungs allow such 
wastes as the gas, carbon dioxide, to be discharged from the 
blood. 
74. Irritability. — In all vertebrates this life process is 
limited to the nervous system, which includes brain, spinal 
cord, nerves, and sense organs as described in the discussion 
of the internal structure of the frog, page 88. Animals 
with definitely developed nervous organs.and a specialized 
brain as in vertebrates are able to do more things than a 
worm, for example. The brain of a frog is really a very simple 
organ when compared with the brain of a dog. This is 
the main reason why a dog can be taught to do so many 
more things than a frog. 
75. Reproduction. — The fundamental process of repro¬ 
duction in the frog family is the same as in all other animals, 
but there is introduced the tadpole stage which makes the 
reproduction of the amphibians different from that of any 
other vertebrate. 
76. Internal Organs. — In order that these several funda¬ 
mental life processes may be better understood in a verte¬ 
brate, ,a knowledge of the structures involved is very 
important. This information will also help you to under¬ 
stand the several organs of man which are discussed in the 
third part of this book. 
Digestive Organs. — The mouth is large. Short lips cover 
the short teeth in the edge of the upper jaw. The tongue, 
which has two fleshy horns at the back end, is attached by the 
front end to the floor of the mouth (Figure 72). The frog 
can throw its sticky tongue over the tip of the lower jaw and 
use the forked end to catch insects which are then carried 
into the back of the mouth. Two groups of little curved 
teeth in the roof of the mouth aid in preventing the escape 
of the prey. The food is swallowed whole. The esophagus 
(the tube connecting the mouth cavity and stomach) of the 
