Body Musculature - Locomotion
The layout of the muscle layers surrounding the coelom is necessarily the same in all the major classes of Annelida. The body wall has a layer of circular muscles beneath the epidermis. This is followed by a layer of longitudinal muscles. In polychaetes the circular muscles are thin and the longitudinal muscles are generally arranged in four blocks, two dorsolateral and two ventrolateral; in some polychaetes oblique muscle strands might extend between the circular muscles of the dorsal and ventral sides in every segment, obdiquely. In oligochaetes, the longitudinal muscles make a continuous layer inner to the circular muscle layer and their long fibers may extend over 2-3 segments, well developed oblique muscles help in localized contraction and expansions of the body segments. In both these groups, the coelom is divided internally through the means of transverse septa. The latter help in resisting the stress caused because of change in the hydrostatic pressure of the coelomic fluid. Of course, in polychaetes, the septa are less developed and broken down, along with coelomic communication between segments. The body musculature is best developed in Hirudinea where, additionally to a double layer of oblique muscles between the circular and longitudinal muscles, there are vertical columns of muscles that are the dorsoventral muscles. Contraction of dorsoventral muscles results in flattening the body of the animal and causes efficient undulations of the body resultant in swimming. But in leeches, coelom is very poorly developed.