Fact 1: Amphibians are the first true tetrapods, or vertebrates with four limbs.

Fact 2: Modern classifications of the tetrapods rely on the structure of the skull .

Given the two facts above, answer the question "what are amphibians classified by?" with the following options: 
- organ
 - ovaries
 - colors
 - feet
 - alleles
 - genus
 - skull
 - Length


 

       skull  