Nerve cord lies in coelom; brain in first segment or prostomium in many forms. Clitellum generally only two or three segments and more anterior in position than in Terricolae.
As a rule - to which, however, there are exceptions - the clitellum consists of two cr three segments only in the small aquatic Oligochaeta, while in the terrestrial forms it is as a general rule, to which again there are exceptions, a more extensive, sometimes much more extensive, region.
There are a clitellum and sperm ducts which though like nephridia have a larger funnel and a less complexly wound duct.
The clitellum consists of a thickening of the epidermis, and is of two forms among the Oligochaeta.
It is therefore obviously much thicker than the clitellum in the limicolous forms. The position of the clitellum, which is universal in occurrence, varies much as does the number of component segments.
The appendages of the body are reduced to branchiae, present in certain forms. A clitellum is present.
It is a remarkable and newly-ascertained fact that in regeneration (in Potamilla) the thorax is not replaced by the growth of uninjured thoracic segments; but that the anterior segments of the abdomen take on the same characters, the setae dropping out and being replaced in accordance with the plan of the setae in the thorax of uninjured worms. Among the Oligochaeta the sexually mature worm is distinguished from the immature worm by the clitellum and by the development of genital setae.