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Ascidians or sea-squirts are sessile tunicates. They are exclusively marine and form a conspicuous part of the fauna, on both sheltered rocky shores and in deeper waters, where they often are fixed in mud and sand by means of fibrils.
All ascidians are filterfeeders; water enters the oral siphon, food particles are filtered out in the branchial sac, and the water leaves the body through the atrial siphon. They are either solitary or colonial. In this last case the individuals are called zooids and are often arranged in systems round common cloacal openings.

- External appearance
In all ascidians the outer surface is covered with a test or tunic, a protective layer of cellulose-like substance, and in the colonial species this forms a common test in which the zooids are embedded.
The body of solitary species ranges from spherical to cylindrical in shape. One end is attached to the substratum, and the other end contains two openings that may be extended as two separate siphons. All shades of colouring are found in ascidians, although grey and green colours are most common. The body ranges in size from a few mm to 15 cm. In colonial species, zooids may arise from stolons or a thick basal mass of common test, or be wholly immersed. The zooids may form firm fleshy lobes or encrustations. In the Didemnidae, some species have minute calcareous spicules in their test. These spicules are produced by lateral thoracic organs.

- Internal morphology
Most of the internal structures are related to the different orders and suborders. Although a special topic for the classification exists, it is inevitable to use the classification in the general description.

Two siphons connect the internal branchial sac to the surrounding environment. The rim of the siphons might be characteristically divided into a number of lobes, related to the mechanism of closure. Pigment spots, which in some species occur at the base of the lobes, might be light-sensitive, as in the case of Ciona, but are not necessarily so, as in the case of Ascidia. The oral siphon varies little throughout the whole group, but the atrial one is strongly modified in the colonial tunicates.

Branchial tentacles
Around the base of the oral siphon, a ring muscle is present, which bears the branchial tentacles. These tentacles prevent large particles from coming in with the water current. In some genera these tentacles are simple, while in others they can also be tapering or crescent-shaped with side branches (simple tentacles, branched tentacles).

Branchial sac
The branchial sac is fundamentally the same throughout the ascidians. It consists of a branchial wall, an endostyle and a peri-pharyngeal band. The branchial wall is perforated by rows of stigmata. The number of rows varies from two to several hundreds. Blood vessels pass between the rows of stigmata and are known as the transverse vessels.
In the suborder Aplousobranchia, zooids are relatively small and the number of rows of stigmata varies between 2 to about 25. No papillae or longitudinal vessels are present, but the transverse vessels give rise to a series of horizontal membranes, ciliated along their edges (except for Ciona intestinalis, which has more rows of stigmata, inner longitudinal vessels and a large tubular papilla at each junction of a longitudinal and transverse vessel).
In the families of the suborder Phlebobranchia another system of vessels is formed on the inner lining of the branchial sac. From the junctions made by the transverse vessels and about every fourth or fifth inter-stigmatic vessel, bifidly branched papillae grow out into the branchial sac. The branches of these papillae unite with similar processes arising from adjacent rows and thus form inner longitudinal vessels. In some families, secondary papillae grow out into the branchial cavity. The stigmata in the suborders belonging to the order Enterogona are all straight (Enterogona stigmata).
In the Pleurogona, the branchial sac with smooth inner longitudinal vessels has further developed. On each side, the branchial sac is thrown into longitudinal folds (longitudinal folds).
On these folds the inner longitudinal vessels lie closer together than between the folds (longitudinal vessels).
Although in many genera a secondary reduction of the folds has taken place, the organization of the inner longitudinal vessels still mark their positions. In certain genera and families of the Pleurogona a further development of the stigmata into curved, and even three-dimensional structures occurs as well (curved stigmata).
In spite of these different developments of the branchial sac, the endostyle has remained a constant structure throughout the ascidians. It is a deep groove along the ventral mid-line of the branchial sac. The endostyle produces mucus, which traps food particles from the water that passes through the branchial sac. Generally, the groove consists in cross-section of three masses of glandular secretory cells on each side, separated by two thinner parts with short cilia and a mid-ventral band with long flagella, protruding into the endostylar lumen (endostyle).
Through ciliary action, the secreted mucus is moved up over the branchial wall to the dorsal lamina. The dorsal lamina is the mid-dorsal region of the branchial sac, opposite the endostyl, which extends to the opening of the oesophagus. It occurs in two forms, as a series of so-called languets and as a continuous membrane. Water is drawn in through the oral siphon by the activity of the stigmatic cilia. The water is then forced through the stigmata into the peribranchial cavity and is released through the atrial siphon. In solitary ascidians the incoming water is taken from a region within a few millimetres of the oral siphon, whereas the filtered water emerging from the atrial siphon is shot out to about 10 times the distance. This causes the two currents to be isolated from one other even in stagnant water (water circulation).

Digestive system
In general, the digestive system is differentiated into an oesophagus, stomach and intestine or rectum. The wall of the stomach may be smooth, with longitudinal folds, or with numerous small blind evaginations (ascidian stomach). A pyloric gland grows out at the basal region of the stomach to form ramifying branches over the intestine. In some families the stomach is virtually lost, but instead a gastric caecum or liver has developed. The food-containing mucus cord is passed through the digestive system by the action of cilia. The anus opens sufficiently near the atrial siphon for the faeces to be removed with the exhalent stream.
Because ascidians lack capillary vessels, no distinction can be made between blood and tissue fluid. Excretion either takes place by special cells circulating in the body fluid, the nephrocytes, or by renal vesicles or a renal organ. In the renal organ, fluids or semisolid materials are permanently stored.