Many species of annelids have the ability to regenerate. These regenerative abilities are greater in the polychaetes and more primitive oligochaetes than in the more specialized oligochaetes. Leeches are unable to regenerate. Most polychaetes and oligochaetes can regenerate a new tail when the body is severed. This ability is usually restricted to the anterior end, where lost segments are replaced either by the same number or by slightly fewer segments. Regeneration can even occur from a single segment, as it does in the polychaetes Ctenodrilus and Dodecaceria.
As in any regeneration, there are several steps. First, the wound seals over, and then a (blastema) forms beneath this surface. The blastema probably arises from the differentiation of existing cells, although mesodermal neoblasts, a totipotent stem cell, migrate to the site of the injury in polychaetes and many oligochaetes. Regenerating structures are formed from this blastema. A hormone from the posterior part of the brain is essential for posterior regeneration.
Some species, such as the small terrestrial oligochaete Enchytraeus japonensis, reproduce asexually by fragmentation and regeneration. The nervous syetem appears to be important in this event. During the fragmentation process, the circular body wall muscles contract near one of these circumferential structures in the middle of the segment. This contraction causes constriction and results in fission of the body. During regeneration, nerve fibers grow from the remaining ventral nerve cord and gradually form networks in both the anterior and posterior regeneration buds. The growing fibers extend to the prostomium (a sensory organ) at the anterior end, prior to connecting to the presumptive brain rudiment. A neural network appears around the pygidium, and this is followed by growth of the body at the posterior end (Myohara et al., 2000).
Laboratory exercises involving the regeneration of earthworm segments can be found in Johnson, 1995.
Johnson, L. G., 1995. Johnson & Volpe's Patterns & Experiments in Developmental Biology, Second Edition. Wm. C. Brown Publishers.
Myohara, M., Yoshida-Noro, C., Kobari, F., and Tochinai S. 2000. Fragmenting oligochaete Enchytraeus japonensis: a new material for regeneration study. Dev. Growth Differ. 41:549-555.