Segmentation in Regenerating Annelids

Several worms as they grow continue to add new segments at the posterior end. In these, segmentation takes place in a growth zone located immediately anterior to the terminal pygidium. It is in this region which extra segments are partitioned off from time to time by the development of new septa. Caudal regeneration recapitulates this common mode of growth in annelids. There is no growth zone at the anterior end to give for increase in segments anteriorly during normal ontogeny. Thus in the - regenerating head, certain number of inter segmental septa are formed at the same time, delimiting certain number of segments. This is why the power of regeneration of anterior segments is usually less than that of the posterior segments.