Crinoids are another great example of animals that look more like plants. Despite their appearance, they are members of the phylum Echinodermata (along with starfish, brittle stars and sea cucumbers) and their branch-like appendages are actually feeding arms, which surround a mouth. There are over 600 species of Crinoid, which can be roughly divided into two groups: Sea Lilies, which are attached to the sea floor by a stem; and Feather Stars, which have no stem. Regardless of which structure they have, many crinoids remain attached to the sea floor only during juvenile stages, and are free swimming in adulthood.
A great video of a swimming crinoid can be found here.