A hypothetical or obscure group of organisms described as having a notochord that is not clearly visible or well-developed.
From Greek 'a-' (without), 'delos' (clear), and 'chorde' (string/cord). The term refers to creatures whose notochord—the structural rod in primitive chordates—is unclear or reduced.
The notochord is basically evolution's first draft of a backbone, and studying creatures with unclear notochords helps scientists understand how the spine evolved in the first place!
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