SEA SQUIRT'S
In the simplest systems, the individual animals are widely separated, but linked together by horizontal connections called stolons, which grow across 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. The body of an ascidiacean is surrounded by a test or tunic, from which the subphylum derives its name. This varies in thickness between species but may be tough, resembling cartilage, thin and delicate, or transparent and gelatinous. The tunic is composed of proteins and complex carbohydrates, and includes tunicin, a variety of cellulose. The tunic is unique among invertebrate exoskeletons in that it can grow as the animal enlarges and does not need to be periodically shed. Inside the tunic is the body wall or mantle composed of connective tissue, muscle fibres, blood vessels, and nerves. Two openings are found in the body wall, the buccal siphon at the top through which water flows into the interior and the atrial siphon on the ventral side through which it is expelled.