free-moving polychaete

… (Polychaeta), which are divided into free-moving and sedentary, or tube-dwelling, forms; the earthworms (Oligochaeta); and the leeches (Hirudinea).

…into subclasses or orders, the Errantiata (free-moving forms) and Sedentaria (sedentary, or tube-dwelling, forms), based on the mode of living. This arrangement, while convenient, is not based on morphology and is not generally used. The classification system given above lists 23 orders (Archiannelida was considered as one order in the…