SEA SQUIRT

Sea squirt also called ascidian any member of the invertebrate class Ascidiacea (subphylum Urochordata, also called Tunicata), marine animals with some primitive vertebrate features. Sea squirts are fixed growing organisms resembling potatoes more than animals; they are found in all seas, from the intertidal zone to the greatest depths. All adult forms are sessile (permanently fixed to a surface). They are commonly found on pier pilings, ships' hulls, rocks, large seashells, and the backs of large crabs. Some species live individually; others live in groups or colonies. The body has an outer protective covering, the tunic. There are two large pores, one to take water into the body cavity, the other to expel it. Food and oxygen are taken from the water current. Near the shore, debris from dead plants and animals constitutes an important part of the diet; in deeper water, plankton (microscopic plants and animals) is a more important food. Although all adults have functional male and female reproductive organs, eggs shed into the water are fertilized by sperm from another individual. The tadpole-like larvae are free-swimming. Behind a sucker on the front of the head is the mouth. A notochord (a flexible, rodlike structure common to all vertebrates) and a nerve cord are in the tail. When the larva finds a place to settle, it attaches itself by the sucker; the tail, with its notochord and nerve cord, is absorbed and disappears. Reproduction also occurs by budding: near the base of the sea squirt fingerlike projections (stolons) break off and settle elsewhere to become new individuals.