Which of the following phyla is characterized by the absence of a coelom?
Platyhelminthes are acoelomates, meaning they lack a body cavity between the body wall and the gut.
Ref: NCERT Class 11 Biology Chapter 4: Animal Kingdom Phylum Arthropoda
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Which type of coelom is present in arthropods?
Arthropods have a hemocoel, where the body cavity is filled with blood and functions as an open circulatory system.
Ref: NCERT Class 11 Biology Chapter 4: Animal Kingdom Basis of Classification - Symmetry Coelom Segmentation
Hemichordates show which type of coelom?
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
Body cavity completely lined by mesoderm is called
Coelom or true body cavity defined by complete lining of mesoderm-derived peritoneum on both outer somatic side lining body wall as parietal layer and inner splanchnic side covering visceral organs as visceral layer, with mesothelium secreting serous fluid cushioning organs and providing hydrostatic skeleton. Contrast pseudocoelom where outer wall mesodermal but gut wall endodermal not peritoneal, blastocoel persistent embryonic cavity not mesodermal, and acoelom solid mesenchyme. Annelids, arthropods secondarily modified, molluscs reduced, chordates prominent exemplify eucoelom providing compartmentalization and mesentery suspension.
Ref: Brusca & Brusca Invertebrates Coelom Definition; NCERT Class 11 Chapter 4 Body Cavity