the main bone, made up of many small bones joined together, that runs along the middle of the back in some animals
a hard outer covering that supports and protects some animals without backbones
to shed an animal's outer covering
organs for breathing found in fish and amphibians
one of a large group of animals with backbones that live part of their lives in water and part on land
an animal with a backbone that has dry, scaly skin
an animal with a backbone that usually has hair on its body and feeds milk to its young
a simple, automatic behavior
the cause of a behavior
a behavior caused by a stimulus
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