Seaweed, the common name for many kinds of marine algae. Although the name is sometimes used for all of the marine algae, most botanists restrict the term to the larger algae that are attached or have been attached to the sea bottom. The brown algae, including the kelps, and the red algae are most commonly called seaweeds. Kelp is used as food and as fertilizer in the Far East. Kelp and red algae are frequently used as animal feed in Europe.
Sometimes you meet someone who smiles at you and exchanges pleasantries, but you still walk away feeling certain that you don't like him or her. You're not nuts -- you might be picking up on microexpressions.
One kind of flatworm doesn't have a working mouth, because the algae that lives inside of it makes food for the worm. Symbiosis is crucial to the survival of many species. In fact, you're a symbiote yourself.