This grouping is part of the Chordata , a phylum which includes all animals with dorsal nerve cords and notochords including vertebrates. The subphylum was at one time called Urochordata , and the term urochordates is still sometimes used for these animals.
Despite their simple appearance and very different adult form, their close relationship to the vertebrates is certain. Both groups are chordates, as evidenced by the fact that during their mobile larval stage, tunicates possess a notochord , a hollow dorsal nerve cord , pharyngeal slits , post-anal tail, and an endostyle.
(mh=zWJKEpSziJ48m2hV)0.jpg "Girls Richardson")
They resemble a tadpole. Tunicates are the only chordates that have lost their myomeric segmentation, with the possible exception of the seriation of the gill slits. Some tunicates live as solitary individuals, but others replicate by budding and become colonies , [ 11 ] each unit being known as a zooid. They are marine filter feeders with a water-filled, sac-like body structure and two tubular openings, known as siphons, through which they draw in and expel water.
During their respiration and feeding, they take in water through the incurrent or inhalant siphon and expel the filtered water through the excurrent or exhalant siphon. Adult ascidian tunicates are sessile , immobile and permanently attached to rocks or other hard surfaces on the ocean floor.
Thaliaceans pyrosomes, doliolids, and salps and larvaceans on the other hand, swim in the pelagic zone of the sea as adults. Various species of ascidians , the most well-known class of tunicates, are commonly known as sea squirts , sea pork, sea livers, or sea tulips.
