The interesting studies are all in zebrafish and salamanders, not worms.
One of the more intriguing of the recent ones is this from last year, strongly reinforcing the theory that most species lost proficient regeneration due to evolutionary battles between cancer and aging. The greater question is perhaps why there are any proficient regenerators among the vertebrates rather than why we can't do it.
https://www.ucsf.edu/news/2015/11/251746/human-gene-prevents...
"Whether the regenerative powers of zebrafish and salamanders represent ancient abilities that mammals have lost, perhaps in exchange for advanced tumor-suppression systems remains an open question for biologists. Most tumor suppressor genes, being extremely useful for preventing cancer and for forming tissues during development, are broadly distributed and conserved across many different species. Recent studies, however, suggest that one, the ARF gene, arose more recently in the avian and mammalian lineage, and has no equivalent in the genomes of highly regenerative animals. To explore whether this gene might play a role in preventing tissue regeneration in humans, the researchers added human ARF to the zebrafish genome and assessed how it affected the fishes' normal ability to regrow damaged fins after injury. They found that human ARF had no effect on the fishes' normal development or response to superficial injury, but when the researchers trimmed off the tip of a fish's tail fin, the gene became strongly activated and almost completely prevented fin regrowth by activating a conserved tumor-blocking pathway."