circular muscle layer

From ZooTerms (Dictionary of Invertebrate Zoology)
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circular muscle layer: 1. The outermost muscular layer.

2. (Bryozoa: Phylactolaemata) In the body, the outer of the two thin muscle layers, between the peritoneum and the epithelium.

3. (Echiura) In all, except for genus Ikeda.

4. (Sipuncula) May be grouped into defined bands or fasicles or formed in sheets.