There are around 1,000 species of barnacle. For many years biologists didn't know much about them. One person to change that was none other than Charles Darwin who spent eight years of his life studying them - research that was to culminate in a two-volume work that remained the definitive word on these animals for nearly one hundred years. You can read about the impact this study had on Darwin's thinking here.
The great Thomas Henry Huxley said this about barnacles: "A crustacean fixed by its head, and kicking the food into its mouth with its legs."
Barnacles are typically attached for life to a rocky substrate but of course there are many exceptions. Some, such as goose barnacles, may settle out as larvae on floating pumice, thereby covering thousands of miles of open ocean. Others attach to whales and travel even further. While most barnacles are filter-feeders this isn't true of all of them. One group have become parasites, particularly of crabs.
Female Sacculina enter a crab through one of its joints and grow inside the crab, extending feeding tendrils throughout the body. Male Sacculina larvae search for females, and, once having found one, deposit cells into a "testis" sac in the female. Here, male cells produce sperm with which the female fertilizes her eggs. There is no male per se - just a few cells producing sperm. The parasite is able to modify male crab behavior to the extent that male crabs will shake barnacle eggs loose into the water in the same way that female crabs release their own eggs.
While some hermaphroditic barnacles release sperm into the water, most utilize their penis to inseminate neighbors - in some species this may be up to 20 times the length of their body - equivalent to a 2m man having a 40m long penis - awkward.
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Ryan Photographic - Cirripedia - Barnacles