Look at what they can do without legs that animals with legs

（主语应该是你或你们 这是祈使句）Look at（谓语） what（宾语） they can do without legs that （what的定语）animals with legs can't do.
they can do without legs 是what引导的宾语从句
that animals with legs can't do.that引导的从句做what的定语