Ascidians, also known as sea squirts, are marine invertebrates belonging to the class Ascidiacea. They are characterized by their sac-like bodies and are typically found attached to rocks or other surfaces in shallow waters. Ascidians filter feed by drawing in water through an incurrent siphon, extracting plankton and organic particles, and expelling the filtered water through an excurrent siphon. These organisms can be solitary or colonial, with some species forming large clusters. Ascidians are important in marine ecosystems as they contribute to nutrient cycling and provide habitat for various marine species. They are also studied for their unique biological features, including their close evolutionary relationship to vertebrates.