Appearance
They look very similar to the sea anemones (Actiniaria), but are not directly related to them. They are solitary, living buried in soft sediments. They live inside and can withdraw into tubes, which are composed of a fibrous material made from secreted mucus and threads of nematocyst-like organelles known as ptychocysts. Within the tubes of the tube anemones, more than one polyp is present, which is an exceptional trait because species that create tube systems usually contain only one polyp per tube.