page_title: Tunicate
section_title: Body form
hierarchical_section_title: Tunicate / Anatomy / Body form
caption_attribution_description: English: A colony of green ascidians from East Timor.
context_page_description: A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata. It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords. The subphylum was at one time called Urochordata, and the term urochordates is still sometimes used for these animals. They 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, 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 siphon and expel the filtered water through the excurrent siphon. Most adult tunicates are sessile, immobile and permanently attached to rocks or other hard surfaces on the ocean floor; others, such as salps, doliolids and pyrosomes, swim in the pelagic zone of the sea as adults.
context_section_description: Colonies of tunicates occur in a range of forms, and vary in the degree to which individual organisms, known as zooids, integrate with one another. In the simplest systems, the individual animals are widely separated, but linked together by horizontal connections called stolons, which grow along the seabed. Other species have the zooids growing closer together in a tuft or clustered together and sharing a common base. The most advanced colonies involve the integration of the zooids into a common structure surrounded by the tunic. These may have separate buccal siphons and a single central atrial siphon and may be organized into larger systems, with hundreds of star-shaped units. Often, the zooids in a colony are tiny but very numerous, and the colonies can form large encrusting or mat-like patches.
caption: Colonial tunicate with multiple openings in a single tunic.
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