A member of the phylum Hemichordata, marine animals that share some characteristics with chordates but lack a true notochord. They include acorn worms and pterobranchs, possessing pharyngeal gill slits and a dorsal nerve cord in some regions.
From Greek 'hemi' meaning half and 'chorda' meaning cord, literally 'half-chordate.' Named in the late 19th century when biologists recognized these animals as having some but not all chordate characteristics, representing a possible evolutionary link.
Hemichordates are like evolutionary 'rough drafts' of chordates - they have pharyngeal gill slits like fish but their 'notochord' is actually just a small forward extension of their gut! Acorn worms can regenerate their entire front end, including their proboscis and collar.
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