|Bryozoans are aquatic invertebrates that can take on many different forms. They are actually a colony of polyps, called zooids, that attach to a common stalk or other hard surface. They can resemble a branching seaweed or coral or they may just encrust a hard substrate leaving an intricate lacy design on its surface. They commonly appear calcified. The distinguishing characteristic of bryozoan zooids is its feeding structure called the lophophore, a ring of ciliated tentacles surrounding a mouth. It may be horseshoe shaped or circular. However, the lophophores are so small, as are the zooids, that they cannot be seen without the help of a microscope and may be withdrawn when not feeding. Phoronids (worm-like animals) and the Brachiopods (bivalve-like animals sometimes referred to as lampshells) also possess lophophores. They are important to the ecosystem because their filter feeding filters out excess food and debris and keeps the water clear. However, they are a major fouling organism on marine structures and are likely to become alien invaders due to dispersion by ships hulls and ballast waters.