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(1) An elastic, unstructured plate situated under the single-layer surface epithelium in some groups of invertebrates (ciliated worms, nemerteans), Hemichordata (Balanoglossus), and Cephalochorda (lancelets). It is formed from the secretions of epithelial cells. It separates the epithelium from the underlying musculature or connective tissue and serves as an attachment for muscles.
(2) In hydroid polyps, the basal plate (corresponds to the mesoglea in medusae) is a delicate unstructured plate lying between the ectoderm and endoderm.