coelenteron

cnidarian anatomy

Also known as: gastrovascular cavity, hollow gut

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June 24, 2026

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Cnidarians, a phylum of over 9,000 species, include animals like corals, jellyfish, and sea anemones. They are characterized by radial symmetry and possess two primary cell layers: an outer ectoderm and an inner endoderm that lines the coelenteron. The coelenteron, a central cavity, serves multiple functions, including digestion, circulation, and waste elimination. Food enters and waste exits through a single opening, the mouth, which leads to the coelenteron. Digestion begins extracellularly in the coelenteron and is completed intracellularly by endodermal cells. Respiration and excretion occur at the cellular level, with cells exchanging gases and wastes directly with the surrounding water.12345

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circulatory system
Animals without independent vascular systems
Their fundamentally simple structure—with a gastrovascular cavity continuous with the external environmental water—allows both the endodermal and ectodermal cells of the body wall access to aerated water, permitting direct diffusion.
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Their fundamentally simple structure—with a gastrovascular cavity continuous with the external environmental water—allows both the endodermal and ectodermal cells of the body wall access to aerated water, permitting direct diffusion.
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invertebrate digestive system
Saccular systems
…digestive cavity is called a gastrovascular cavity, because in many animals it has vessel-like branches that convey the contents to all parts of the body.
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…digestive cavity is called a gastrovascular cavity, because in many animals it has vessel-like branches that convey the contents to all parts of the body.
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