Any elongated, flexible, fleshy appendage that is not supported by bones or any sort of rigid skeleton might be called a tentacle, says Stuart Blackman.
What's the point of tentacles?
Powered by muscles working against internal fluid pressure, tentacles are used variously as sensory
organs, to catch and manipulate food, and in locomotion.
Perhaps the archetypal tentacles are those that encircle the mouths of cephalopod molluscs, such as squid, octopuses and cuttlefish, which are equipped with suckers that help grasp prey and other objects. In octopuses, they are also used as walking legs. Other molluscs have tentacles, too. Two pairs, one of which bears eyes on the tips, can be seen on the heads of slugs and snails.
Tentacles are also a characteristic feature of jellyfish, sea cucumbers, anemones and other cnidarians, and are often armed with stinging cells that immobilise prey. The lion’s mane jellyfish (below) is a contender for the longest animal on Earth thanks to its 30m-long tentacles.
Strangely, we don’t think of our tongue or an elephant’s trunk as tentacles, even though both tick the boxes. The term is generally reserved for structures found in invertebrates, though the fleshy protuberances surrounding the face of a star-nosed mole (one of the world's weirdest animals) are a notable exception.