Larvaceans, class Appendicularia, are solitary, free-swimming tunicates found throughout the world's oceans. Like most tunicates, larvaceans are filter feeders. Unlike most other tunicates, they live in the pelagic zone, specifically in the photic zone, or sometimes deeper. They are transparent planktonic animals, generally less than in body length, excluding the tail. Anatomy The adult larvaceans resemble the tadpole-like larvae of most tunicates. Like a common tunicate larva, the adult Appendicularia have a discrete trunk and tail. Larvaceans produce a "house" made of mucopolysaccharides and cellulose. In most species, the house surrounds the animal like a bubble. Even for species in which the house does not completely surround the body, such as Fritillaria, the house is always present and attached to at least one surface. These houses are discarded and replaced regularly as the animal grows in size and its filters become clogged; in Oikopleura, a house is kept for no more than four hours before being replaced. No other tunicate is able to abandon its test in this fashion. Discarded larvacean houses account for a significant fraction of organic material descending to the ocean depths. The tail of larvaceans contain a central notochord, a dorsal nerve cord, and a series of striated muscle bands enveloped either by epithelial tissue (oikopleurids) or by an acellular basement membrane (fritillarids). Unlike the Ascidiacea larvae, the tail nerve cord in larvaceans contain some neurons. As the larvae of ascidian tunicates don't feed at all, the larvae of doliolids goes through their metamorphosis while still inside the egg, and salps and pyrosomes have both lost the larval stage, it makes the larvaceans the only tunicates that feed and has fully functional internal organs during their tailed "tadpole stage", which in Larvacea is permanent. Feeding Larvaceans have greatly improved the efficiency of food intake by producing a test, which contains a complicated arrangement of filters that allow food in the surrounding water to be brought in and concentrated prior to feeding. By regularly beating the tail, the larvacean can generate water currents within its house that allow the concentration of food. The high efficiency of this method allows larvaceans to feed on much smaller nanoplankton than most other filter feeders. Like most tunicates, larvaceans feed by drawing particulate food matter into their pharyngobranchial region, where food particles are trapped on a mucus mesh produced by the pharynx and drawn into the digestive tract. The mucus mesh