The tunicates are favored as vertebrate ancestors in the long past. Nothing can be seen of a vertebrate relationship in the adult animals, but the tadpole larva has a so called notochord, a stiff rod in its tail. It is because of the imagined evolutionary position as vertebrate ancestors, that these interesting small animals have excited such a wide interest.
Tunicates are very interesting small marine animals.
In essence the animal consists of a double sac with two siphons. Sea water is pumped slowly through one siphon, sieved through the inner sac for plankton and organic detritus , and the filtered seawater is pumped out again through the second siphon, the atrium. The pumping is caused by cilia. The current through the atrium is faster than the incoming one, to avoid mixing of the two currents. The atrium also serves as an exit for faeces and sex cells.