THE AMPHIBIANS 179 
tation. Their shrill, cricket-like calls are often heard in 
the summer. The fingers and toes are more or less dilated 
into disks at their tips, enabling them to climb with con- 
siderable facility; and they are further adapted to their 
surroundings on account of their protective colors. The 
toads undergo their metamorphosis while very small, and 
approach only the water at the breeding season. During 
the day they remain concealed in holes and crevices, but at 
the approach of evening come out in search of food. 
171. Means of defense.—The food of the members of this 
group consists chiefly of small fishes, insect larve, snails, 
and little crustaceans, which are swallowed whole. On the 
other hand, many Amphibia prey on each other, while most 
of them are eagerly sought by birds and fishes. Some, as 
the toads, stalk their food only during the night-time or 
depend upon their agility to escape their enemies. Others 
are colored protectively, the markings of the skin resem- 
bling the foliage of the earth upon which they rest, and in 
some species, as the tree-toads, this color-pattern changes 
as the animal shifts its position. A few species are most 
brilliantly colored with red, green, yellow, or combinations 
of these, in striking contrast to their surroundings. They 
have apparently few enemies, possibly because of an un- 
pleasant odor or taste, and it has been suggested that their 
gorgeous tints are danger-signals, warning their would-be 
captors from attempting a second time to devour them. At 
the same time it is well known that the somber-hued toads 
emit a milky secretion from the warty protuberance of 
their skin which is intensely bitter, irritating to delicate 
skin, and poisonous to several animals. 
172. Skeleton.— As in all vertebrates, the skeleton of the 
amphibian first arises as a cartilaginous rod, the notochord, 
which is afterward replaced by a jointed back-bone, to 
which the limbs are attached. The back-bone is anteriorly 
modified into a flat, usually complex, skull. In the sala- 
manders the number of vertebre is sometimes very large, 
