CONTROL OF REGENERATION IN ANNELIDS - EXA MPLE OF NEREIDS (ANNELIDA POLYCHAETA)

In Nereidae (Annelida polychaeta), regeneration is under the influence of nervous system and tissue interactions. Caudal regeneration is acc omplished by restoration of the pygidium, followed by the appearance o f metameres between the regenerated pygidium and the last segment of t he stump. Regeneration of the pygidium is initiated by contact between the intestine and the bodywall; metamere regeneration results from th e activity of the prepygidial proliferation zone which restores the pr oximo-distal gradient of segmental differentiation under endocrine con trol of cerebroid ganglions. The injured nerve cord releases a trophic factor which stimulates activation of blastemal cells. Regeneration o f parapodia is initiated by the juxtaposition of the dorsal body wall and the ventral body wall in the presence of a parapodial nerve. Deter mination of the dorso-ventral axis of the metameres is controlled by t he nerve cord which ''ventralizes'' the territory in its close vicinit y.