The inhalant siphon is used to take in food and water, and the exhalant siphon expels waste and water.
The tunic is a thick, usually opaque, covering protecting their barrel-shaped bodies from predators. It is made from a material very similar to cellulose. On the inner surface of the tunic is a thin epidermis, it is this which secretes the tunic. On the inside of the epidermis is a thicker dermis (body wall) and bands of muscles which can squeeze the tunic forcing a jet of water from the exhalant siphon.
Most of the space within the tunicate body is taken up by the atrium (a large cavity). This contains the enlarged pharynx, which has large numbers of small pores or slits in its walls through which water can pass. It is connected to the digestive system at one end and to the inhalant siphon at the other, this is effectively the animal's mouth.
The tunicate's pharynx is covered by tiny hairs (ciliate cells) which allow the consumed food to pass down through to the oesophagus. The digestive system is U-shaped, the anus emptying directly to the outside.
Tunicates are filter feeders, feeding by drawing often hundreds of litres of water each day through the inhalant siphon. This water passes through the pharynx where small particles are filtered out before the water is expelled through the exhalent siphon. The water current is caused by beating cilia. Water can also be pushed out of the atrial cavity by muscular contractions of the tunic if the tunicate is threatened. The small particles of plankton, etc, are trapped on a continually moving layer of mucous. This mucous is released by special cells and is moved across the surface of the pharynx by the beating of many small cilia, until it is passes into the digestive system where the food particles and mucous are digested.
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