A numerous and widespread group of planktonic molluscs is the Pteropoda, small opisthobranch gastropods. These are of two types, the tiny thecosomatous forms which have a very lightly built shell, for example Spiratella (Limacina) (Figure 2.14h) and the larger gymnosomatous forms which have no shell, for example the sea butterfly Clione. The latter is large enough to be seen regularly by divers. Various pelagic prosobranchs occur in warm oceans for example Carinaria, Pterotrachea and Janthina. These have light, fragile shells to assist floatation. Janthina, the violet snail, is holopelagic, drifting at the surface of the sea by means of a float of mucus mixed with air bubbles. It feeds on the pelagic coelenterates, Porpita and Velella, the by-the-wind sailor. Benthic molluscs produce innumerable planktonic larvae.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.