a class of Chordata of the subphylum Tunicata. The saclike body of the Ascidiae is fastened to the seabed by its lower end and has two openings on its upper end—oral and cloacal. The body of the Ascidiae is clothed in a tunic, a thick membrane that has an epithelium and contains animal cellulose (tunicin). The pharynx opens by gill slits into the peribranchial cavities, which are connected with the cloaca.
There are about 150 species of Ascidiae, widely distributed in seas all over the world. They feed upon small organisms and detritus that they filter out of the water. Ascidiae are hermaphrodites. They reproduce sexually and by budding, which often leads to the formation of colonies. There are both simple—or individual—Ascidiae (Monas-cidiae), and complex—or colonial—Ascidiae (Synascidiae). The development of the Ascidiae includes a stage of life as a free-floating, tailed larva with a notochord, which adult Ascidiae lack. These peculiarities in the structure of the larva permitted the Russian scientist A. O. Kovalevskii to show that Ascidiae belong to the phylum Chordata (1866).