After 8-1 0 segments of posterior ventral nerve cord were ablated in Lumbriculus variegatus, touch-evoked locomotor responses were evident both in segments anterior and posterior to the ablation site. However, responses in these two regions were independent and uncoupled. During recovery, four outcomes were observed at the ablation site: (Group 1) recovery of normal functions with no growth of new segments; (Group 2) formation of a laterally protruding, multi-segmented, ectopic head; (Group 3 ) formation of a laterally protruding, amorphous, and multi-segmented outgrowth; and (Group 4) segmental autotomy. i n Groups 1 and 2, touch-evoked swimming and body reversal were studied. in addition, sensory fields and conduction properties of giant nerve fibers were examined near the ablation site. i n some Group 1 worms, clear-cut behavioral and electrical signs of recovery and reconnection were seen by 3 d after ablation. By 8 d, all worms had recovered and exhibited response patterns comparable to those of normal worms. i n Group 2 worms, with an ectopic head, segments posterior to the ablation (together with those in the ectopic head), exhibited touch-evoked swimming and body reversal responses resembling those of a complete worm. Segments anterior to the ectopic head were independently capable of locomotor responses. Medial and lateral giant fiber sensory fields in worms with ectopic heads reflected a pattern expected for two worms. Thus, through apparent morphallactic reorganization, a medial giant fiber sensory field emerged which included the ectopic head and 10-1 5 adjacent posterior segments. In contrast, electrical recordings showed longitudinal through-conduction of giant fiber spikes, across the ablation site. Histological examination revealed that the giant nerve fibers in the ectopic head were complexly interconnected with those in the main body axis.Additional key words: giant axons, ectopic head, neural plasticity

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Access to the ventral nerve cord in living specimens of Lumbriculus variegatus, an aquatic oligochaete, is normally impossible because surgical invasion induces segmental autotomy (self-fragmentation). We show here that nicotine is a powerful paralytic agent that reversibly immobilizes worms, blocks segmental autotomy, and allows experimental access to the nerve cord. Using nicotine-treated worms, we transected the ventral nerve cord and used non-invasive electrophysiological recordings and behavioral analyses to characterize the functional recovery of giant nerve fibers and other reflex pathways. Initially, after transection, medial giant fiber (MGF) and lateral giant fiber (LGF) spikes conducted up to, but not across, the transection site. Reestablishment of MGF and LGF through-conduction across the transection site occurred as early as 10 h (usually by 20 h) after transection. Analyses of non-giantmediated behavioral responses (i.e., helical swimming and body reversal) were also made following nerve cord transection. Immediately after transection, functional reorganization ol' touch-evoked locomotor reflexes occurred, so that the two portions of the worm anterior and posterior to the transection site were independently capable of helical swimming and body reversal responses. Similar reorganization of responses occurred in amputated body fragments.

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