Hydrothermal vents, those oases of the barren deep-sea floor, are home to an animal you have probably seen before, the giant tube worm:
Aside from populating the sunless ocean floor, these worms are remarkable for another reason: they have no digestive tract. Instead, giant tube worms rely on a symbiotic relationship with bacteria.
In a process known as chemosynthesis (think photosynthesis but with chemicals as the source), the brilliant red plumes of the giant tube worm collect nutrients spewing from nearby hydrothermal vents, such as oxygen, hydrogen sulfide, and carbon dioxide, and feed the bacteria living within the worms’ trophosome. The bacteria use up these nutrients and covert them into organic molecules that the worms can then feed on, no digestive system required.
[Via Wikipedia, NOAA]