Two Japanese researchers found that a substantial proportion (33%) of two species of sea slugs (Elysia cf. marginata and E. atroviridis) were observed to shed their own heads (“autotomy”, a fancy word for “self amputation”) in the laboratory. Moreover, the heads regenerated new bodies—and quite quickly: within 20 days. The shed bodies, which did not regenerate new heads but died, contained the heart and the digestive systems. The heads, meanwhile, closed the wound from “voluntary” separation, began eating algae within hours, and the regeneration of the entire body was complete within 20 days.
Here’s a shot of four phases of the autotomy from the paper (as is the caption):
A) Head and body of Elysia cf. marginata (individual no. 1) just after autotomy (day 0), with the pericardium (heart) remaining in body section (arrow). (B) day 7, (C) day 14, (D) day 22, showing whole-body regeneration.