Many animals in this section start their lives as a larval stage, drifting as part of the plankton. At some stage, often in the Spring, they are triggered to move to their second stage and adopt a static life style. To do this the larva needs to find a hard surface on which to establish itself. This is obviously a very chancy affair and countless billions must fail. Once established, being static animals, they need food to be brought to them and mostly they therefore feed on plankton and suspended organic matter. Sponges, mussels and sea squirts for instance take in water, extract any food from it and then discharge the water. Hydroids tend to rely on tiny tentacles that passing particles stick to.Some static animals are very short lived, lasting a few weeks or months before vanishing until a new generation appears the following year. By contrast, corals, some anemones and sponges once established can live for many years.