Ascidians are sessile marine invertebrates commonly called sea squirts or tunicates because they filter and squirt water through siphons in their tunics. Some 2000 species populate the oceans by travelling on boat hulls. Ascidians are hermaphrodites and comprise most of the urochordate phylum, which includes two other classes: larvaceans (appendicularians) and thaliaceans (salps). Several species of ascidian are used for research: Ciona, Phallusia, Halocynthia, Ascidiella, Styela. Ascidian embyros develop like vertebrate embryos into simple tadpoles made of only 3000 cells and 6 tissues. After 2 or 3 days they attach to a surface and metamorphose into a juvenile that becomes a sessile adult in 2-3 months.They have been a favorite of embryologists for over a century, ever since Conklin described the lineage and discovered myoplasm, a cortical region of the egg that becomes the tail muscles of the swimming tadpole.
The genome of Ciona intestinalis has now be sequenced (the 6th metazoan genome) and ciona is adopted as a developmental model by an increasing number of laboratories.