What characteristic must an animal exhibit at some developmental stage to belong to Phylum Chordata?

Answer

A notochord, a dorsal nerve cord, and pharyngeal slits

Membership in Phylum Chordata signifies that the animal possessed a notochord, a dorsal nerve cord, and pharyngeal slits at some point in its development.

What characteristic must an animal exhibit at some developmental stage to belong to Phylum Chordata?
taxonomysealPinnipedmarine mammalElephant seal