Why do people used to think that sea anemone was a plant

Ответ:

Anemones, or sea anemones [2] (Actiniaria), are a detachment of sea cnidarians from the class of coral polyps (Anthozoa). Representatives are deprived of a mineral skeleton. As a rule, single forms. Most sea anemones are sessile organisms that live on solid seabed. Few species (for example, Nematostella vectensis) are characterized by a burrowing lifestyle in the bottom sediments.