The two nerve-rings are connected by fibres passing from one to the other.
This is the typical arrangement, which is exhibited in the majority of the Polychaeta and Oligochaeta; in these the successive chambers of the coelom are separated by the intersegmental septa, sheets of muscle fibres extending from the body wall to the gut and thus forming partitions across the body.
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Below this is a circular, and below that again a longitudinal, layer of muscle fibres.
The muscular layers are thinner in the aquatic forms, which possess only a single row of longitudinal fibres, or (Enchytraeidae) two layers.