What structure must be present at some developmental stage for placement in Phylum Chordata?
Answer
A dorsal hollow nerve cord or notochord
Placement within the Phylum Chordata is determined by the presence of four fundamental features at some stage during the organism's development. These defining characteristics include a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Although adult sharks do not possess the bony vertebral column characteristic of mammals, they fulfill the structural requirements for this group by possessing a cartilaginous one, linking them evolutionarily to humans and all other vertebrates through these shared, ancient chordate blueprints.
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