Ascidiacea (commonly known as the ascidians or sea squirts) is a class in the Tunicata subphylum of sac-like marine invertebrate filter feeders. Sea squirts are sessile animals: they remain firmly attached to substratum, such as rocks and shells. There are 2,300 species of ascidians and three main types: solitary ascidians, social ascidians that form clumped communities by attaching at their bases, and compound ascidians that consist of many small individuals forming colonies up to several meters in diameter. The lancelets, also known as amphioxi (singular, amphioxus), comprise some 32 species of fish-like marine chordates with a global distribution in shallow temperate and tropical seas, usually found half-buried in sand. Amphioxus is an obsolete synonym of the genus Branchiostoma. It is used as a common name along with lancelet. Undated, no illustrator credited.