8 public questions tagged with this topic.
The stomochord in hemichordates is a rod-like structure that functions similarly to the notochord.
Ref: NCERT Class 11 Biology Chapter 4: Animal Kingdom Phylum Hemichordata and Chordata - Protochordata
Hemichordates have a stomochord, which is similar in function but not homologous to the notochord.
Ref: NCERT Class 11 Biology Chapter 4: Animal Kingdom Phylum Hemichordata and Chordata - Protochordata
Hemichordates do not have a true notochord; instead, they possess a stomochord, which is similar in function.
Ref: NCERT Class 11 Biology Chapter 4: Animal Kingdom Phylum Hemichordata and Chordata - Protochordata
Hemichordates possess a stomochord in the collar region, which is similar in function to the notochord but structurally different. This places them between non-chordates and chordates.
Ref: NCERT Class 11 Biology Chapter 4: Animal Kingdom Basis of Classification - Symmetry Coelom Segmentation
Excretory apparatus in Hemichordata centers on glomerulus or proboscis gland, a vascular knot of blood vessels projecting into protocoel anterior to stomochord, filtering hemolymph through podocytes to produce urine collected via protocoel pore. Additionally, gill slit region contributes to excretion via branchial epithelium diffusion. Among simplified textbook choices, gill slits region is historically linked to excretory function because filtrate drains near pharyngeal area and wastes diffuse across gill surfaces. Unlike segmented nephridia of annelids, intracellular flame cells of flatworms or Malpighian tubules of insects, hemichordate ultrafiltration via glomerulus illustrates primitive deuterostome arrangement.
Ref: Hickman et al., Integrated Principles of Zoology, 17th ed., Hemichordata Excretion; Brusca & Brusca, Invertebrates
Hemichordate coelom development is enterocoelous, characteristic of deuterostomes, where coelomic cavities form as outpocketings of archenteron endoderm that pinch off and expand. This yields three successive compartments: protocoel in proboscis, mesocoel in collar and metacoel in trunk, each lined by peritoneal mesothelium. Enterocoely contrasts with schizocoely seen in protostomes where mesodermal bands split, with pseudocoely lacking complete lining and acoely lacking cavity. Presence of enterocoely places hemichordates alongside echinoderms and chordates within Deuterostomia, supporting embryonic cleavage pattern as major phylogenetic criterion in animal systematics.
Ref: Campbell Biology, 12th ed., Chapter 32: Animal Development – Enterocoelom; Barnes et al., Chapter 22
Hemichordate organization illustrates tripartite coelomic body plan reflecting enterocoelous deuterostome development. Anterior protosome or proboscis contains protocoel and stomochord, functioning in burrowing, exploration and mucus-assisted feeding with independent proboscis pore for coelomic fluid. Middle mesosome or collar contains mesocoel, ventral mouth, dorsal hollow collar cord and neurocord. Posterior metasome or trunk is elongated containing metacoel, numerous paired U-shaped pharyngeal gill slits dorsolaterally, hepatic caeca and gonads. Proboscis-collar-trunk division contrasts with head-thorax-abdomen of arthropods and cephalothorax-abdomen of chelicerates, defining functional specialization for feeding taxonomy.
Ref: Hickman et al., Integrated Principles of Zoology, Chapter 22: Hemichordata Body Plan; NCERT Class 11, Chapter 4
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
