Any tunicate in the class Ascidiacea; found in seas worldwide. Resembling potatoes more than animals, they are permanently fixed to a surface. A forceful contraction of the adult's vaselike body shoots a jet of seawater when it is disturbed. They filter-feed near the shore on debris from dead plants and animals and, in deeper water, on plankton. Adults, from less than 1 in. to 1 ft (2–30 cm) long, have functional male and female reproductive organs. The free-swimming, tadpolelike larvae hatch from eggs shed by one individual and fertilized by another. Some species live individually, others in colonies.

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