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Why are tunicates classified as chordates even though most adults lack a notochord?

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A translucent X-ray fish (Pristella maxillaris) swims in an aquarium, showing its visible backbone and internal structures through its body. — Chordate X-ray fish (*Pristella maxillaris*), is an example of a chordate with a visible backbone.

Tunicates are classified as members of the phylum Chordata because they possess defining chordate characteristics—including a notochord, a dorsal nerve cord, and a tail—during their larval stage.123

While these features are largely absent in most adult tunicates, the classification is based on the presence of these traits at some point in the animal's life cycle. In the tunicate tadpole larva, the notochord provides essential structural support for the muscular tail, which the larva uses to swim and locate a suitable habitat.423

Upon metamorphosis, most tunicates become sessile and undergo significant physical changes, during which the notochord, nerve cord, and tail are typically resorbed. Because these diagnostic chordate features are present during the developmental phase, tunicates remain firmly categorized within the phylum Chordata.12345