Electron micrograph: Claire J. Healy
That's Rhinebothrium sp., a species of tapeworm that parasitize stingrays. It was recently discovered by Claire J. Healy of the Royal Ontario Museum in Toronto and her colleagues.
Attention hypochondriacs: the next time you suffer from abdominal pain, nausea, vomiting, loss of appetite and malnutrition, it may just be one of this fellow's human-infecting cousins living in your intestines. They can grow up to 30 feet long, y'know.