Which organisms are classified under the Chordata phylum?

Organisms classified under the Chordata phylum are characterized by possessing certain key features at some stage of their development, such as a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Chordata encompasses a diverse range of animals, primarily including all vertebrates—fish, amphibians, reptiles, birds, and mammals. These vertebrates exhibit the defining chordate traits, particularly during their embryonic stages.

In contrast, sea cucumbers, clams, and hydra do not have these defining characteristics of chordates. Sea cucumbers belong to the echinoderms, which lack a notochord and other cardinal features of the Chordata. Clams, classified as mollusks, also do not exhibit any of the hallmark features of chordates. Hydra, a member of the cnidarians, is quite different in structure and does not possess the traits that define the Chordata phylum. Thus, the only option that correctly represents organisms classified under Chordata is all vertebrates, affirming the distinction of this phylum within the animal kingdom.