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The Subphylum Tunicata (Urochordata) contains about 1,600 species of marine animals called tunicates, or sea squirts (plus some other strange forms). The name tunicate comes from the cellulose-containing tunic that surrounds the animal. In terms of form and function, tunicates have either spherical or cylindrical bodies that are attached to the substrate by a base or stalk. On the outside are two projections, an incurrent siphon that brings water into a pharyngeal chamber and an excurrent siphon through which the water is expelled. Feeding depends on the formation of a mucous net that traps plankton that are transported to the stomach for digestion. Although adult sessile tunicates lack most of the chordate characteristics, the tunicate larva (sometimes called "tadpole larva") reveals its chordate heritage with its well-developed notochord, propulsive tail, dorsal-tubular nerve cord plus a brain, balancing organ and an eye, complete with lens! |
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