The decapitated heads of certain sea slugs can crawl around as the animal grows itself a full new body. Ecologists from Nara Women's University iun Japan observed that Elysia cf. marginata sea slugs that pull their heads off their own bodies to free themselves of parasites. From Science News:
[Researcher Sayaka] Mitoh first noticed the sea slugs' extreme regeneration by chance in some Elysia slugs in the lab. "We were really surprised to see the head crawling," [Mitoh's colleague Yoichi] Yusa says[…]
On close inspection, the researchers found that sea slugs have a slight groove looped on the back of the head region that seems to work as a break-here zone. The bodies left behind can still move on their own for days or even months. An abandoned body, however, doesn't regrow its head. The leaf-shaped remnant instead turns pale and weak and eventually dies.