Tunicates, vertebrates and lancelets are chordates, members of the animal phylum Chordata. All chordates have, at some point in their lives, a structure resembling a spinal cord, according to Wildlife Journal Junior. This can be a notochord, which is present in the lancelet throughout its life but disappears in the adult tunicate.
Lancelets are small fishlike animals that have a dorsal notochord that supports the growth of their bodies through adulthood. Tunicates grow a notochord during their juvenile phase but lose it on reaching the sedentary adult phase of their lives. Vertebrates do not have notochords as adults, though the structure is found during embryonic development. The notochord of vertebrate embryos eventually gives way to a fully formed spinal column that is often encased in bone, according to Wildlife Journal Junior.
Tunicates and lancelets have no bones at any stage of their lives, though lancelets do have rudimentary skeletons made of cartilage. Of the three groups, only tunicates show a marked divergence between juvenile and adult forms, as they pass through a distinct metamorphosis from free-swimming larvae to stationary adults. Their inclusion in Chordata reflects a phylogenetic relationship based on common ancestry, rather than a functional grouping of similar adult forms.