There is an open blood system with a tubular heart, which lies below the digestive loop. The animal has a simple nervous system. The outer sac, the test, can be leathery or translucent, or the gelatinous or viscous matrix of a colonial, compound form. The test is often brightly colored. The size of the animals mostly ranges from a few millimeters to several centimeters, in N.W. Europe at most 15 cm.
Most Ascidian families are cosmopolitan.
In essence they are one year organisms, developing the first year, then breeding and dying the following year. Ascidians are hermaphroditic. Most large solitary species shed high numbers of weakly yolked eggs into the sea, coincidentally with the release of sperm from other individuals. External fertilization is followed by the development of a free swimming tadpole larva, rarely more than 5mm long. However, there is a great deal of variation in the reproductive strategies of tunicates.