Which feature is unique to phylum Hemichordata?
Hemichordates have a body divided into three parts: proboscis, collar, and trunk.
Ref: NCERT Class 11 Biology Chapter 4: Animal Kingdom Phylum Hemichordata and Chordata - Protochordata
#stomochord
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Hemichordates possess a structure similar to notochord called
Hemichordates exhibit buccal diverticulum extending anteriorly from roof of pharynx into proboscis, called stomochord, historically interpreted as notochord homologue due to vacuolated cells and supportive function beneath heart and glomerular complex. However, developmental origin differs: stomochord derives from endoderm of gut, whereas true notochord originates from chordamesoderm, and it does not express classical notochordal genes throughout. Current phylogeny places Hemichordata with Echinodermata as Ambulacraria within Deuterostomia. Stomochord therefore represents convergent skeletal support aiding proboscis turgor, not homologous axial skeleton, yet signifies deuterostome affinity.
Ref: NCBI Bookshelf, Deuterostomes – Hemichordata; Campbell Biology, 12th ed., Chapter 34: Hemichordates, Stomochord