THE AMPHIBIANS 187 



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 the water only 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. 



177. Means of defense. The food of the members of this 

 group consists chiefly of small fishes, insect larvae, 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. 



178. 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 vertebrae is sometimes very large, 



