The protractor muscles of the bristle-like chaetae of serpulids and sabellids consist of discrete fibres. Each fibre consists of a hollow cylinder of unstriped fibrils surrounded by a sheath of cytoplasm which is expanded into two or more frilled membranes extending along the fibre. The single nucleus lies in one of these membranes. The fibre is apparently ensheathed by a sarcolemma of fine connective tissue-fibres. Similar frilled membranes are present on the muscle-fibres in the walls of the ‘hearts’ of Lumbricus terrestris.

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