Clitellum not present as a definite organ, as in Oligochaeta.
Clitellum 37 always present, extendin over two (many limicolous forms) to forty-five seg ments (Alma).
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.
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.
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.
Clitellum commonly extensive and more posterior in position than in other groups.
The appendages of the body are reduced to branchiae, present in certain forms. A clitellum is present.