Plankton are a diverse collection of organisms that live in water and are unable to swim or have limited swimming ability. Most plankton are tiny, have a spherical, oblate or dendritic form or long protruding spines, and are generally radially symmetrical. A skeleton is not developed or is in the form of a thin shell. Motile organs are not well developed or missing. Their distribution is mainly controlled by the current flow. Examples are foraminifera, radiolaria and planktonic algae. Plankton are usually very abundant species and have a wide distribution. They can be divided into zooplankton and phytoplankton or be categorized into holoplankton, meroplankton and pseudoplankton. They can also be classified based on their size, but the classification criteria are inconsistent.