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Subphylum Urochordata (tunicates)


Ascidian (sea squirt), from Tomiyama et. al., 1970.
Urochordates are found in a wide variety of body forms, lifestyles, and habitats. All are filter feeders, however, some tend to be benthic and sessile (such as the ascidians ), others are pelagic and form important components of the zooplankton (the salps and the appendicularians ). Some are solitary, others are colonial to various degrees. They share the basic characteristics of other chordates , especially the "tadpole-like " larvae, however, the adult forms often lack these and little resemble other chordates .
Class Ascidiacea


Ciona intestinalis, class Ascidiacea , from Tomiyama et al., 1970.
The Ascidiacea is the largest of the three Urochordate classes, and contains the most diversity of body forms. They can be either solitary, social, or compound, with the latter category consisting of numerous small individual "zooids" that are contained within an enveloping gelatinous matrix (which can sometimes be difficult to distinguish from a sponge ).

Ascidians typically have a U-shaped gut, with the incurrent oral and excurrent atrial siphons extended into the water column. Water is drawn into a large chamber (the atrium) by the action of cilia , and food particles are removed by a mucous sheet that covers the pharyngeal basket.

Oikopleura sp., Modified from Alldredge, 1976.

Oikoplera longicauda modified from Tomiyama et. al., 1970.
Class Appendicularia
One of the classes of the subphylum Urochordata , the Appendicularians (also known as Larvacea ) comprise about 70 species of highly specialized urochordates . They retain many larval features, including a muscular tail, a notochord , as well as a hollow dorsal nerve chord .

Appendicularians only reproduce sexually. The free-swimming tadpole larva metamorphoses into a neotenic adult that does not settle but assumes a permanently planktonic existence. Instead of being enclosed in a tunic like the ascidians , they secrete a gelatinous structure (a house) through which they drive water currents by beating of their tail. Water is drawn into the house through a fine meshwork that is part of the house's structure, or in related gelatinous meshworks secreted by the animal. Any fine organic material retained on the mesh is used for food. The water current exiting the house provides some locomotory force. The fine mesh of these filters is capable of filtering out small nannoplankton much more efficiently than that possible by most other filter feeding zooplankton .

Appendicularians are commonly found in the plankton of pelagic surface waters (epipelagic zone), and can ocassionally form extremely dense "blooms " in the water. They may be found in all oceans, including the arctic and Antarctic regions.
Class Thaliacea


Salpa sp., class Thaliacea , from Bianco , 1904.
Thaliaceans are usually gelatinous , transparent urochordates , - which are more common in tropical and subtropical waters (however, there are species inhabiting the polar and temperate regions). Although usually found in coastal and shallow waters, they have been found in water as deep as 1500 meters. Thaliaceans roughly resemble ascideans , with the atrial and buchal siphons at opposite ends of their bodies. As they draw water through there bodies, filtering out small organisms and particles for food, the exhalant current serves as a sort of jet propulsion, which enables them to move slowly through the water.


References:

Alldredge, A., 1976.
Barnes, 1980.
Barnes, Calow, and Olive, 1993
Meglitsch & Schram, 1991
Brusca & Brusca, 1990.
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