The star ascidian is a colonial sea-squirt (2). The individual members of the colony (known as zooids) are embedded in a jelly-like coating called a 'test' in groups of 3-12 around a common opening; this arrangement gives rise to the star-like patterns referred to by the common name (3). Sea-squirts earn their common name because they expel a jet of water when disturbed (3). They are also known as 'tunicates' due to the tunic-like test of many species. The colour of colonies is variable, but includes blue, brown and yellow; furthermore the colour of the zooids often contrasts with that of the test (3). Sea-squirts belong to the same phylum as humans, the 'chordates'. The free-swimming larval stage is a 'tadpole larva', which possesses a stiff chord known as a 'notochord' as well as a nerve chord, which pass along the flexible tail; the possession of both a notochord and a nerve chord at some point in the lifecycle of an animal is a crucial defining characteristic shared by all chordates (3).
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