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.
It also distinguishes the true cotton from the silk cottons or flosses, the fibres of which have no twist, and do not readily spin into thread, and for this reason, amongst others, are very considerably less important as textile fibres.
In the hydropolyp the ectodermal muscle-fibres are always directed longitudinally.
The muscle-fibres arise as processes from the bases of the epithelial cells; such cells may individually become sub-epithelial in position, as in the polyp; or, in places where muscular tissue is greatly developed, as in the velum or sub-umbrella, the entire muscular epithelium may be thrown into folds in order to increase its surface, so that a deeper sub-epithelial muscular layer becomes separated completely from a more superficial bodyepithelium.