Sea squirts, or ascidians, are a diverse class of marine invertebrates representing about 2,300 species. As adults they are sessile animals (attached to the bottom) that feed by filtering food from water, sea squirts exhibit three main types: solitary, social, and compound. Social sea squirts attach at their bases to form clumped communities of genetically identical clones. Compound sea squirts (like the siphonophores) form colonies by budding off many small, genetically identical and interconnected individuals called zooids. Unlike the adults, the larvae of sea squirts can swim and have a structure that represents an early stage in the evolution of the backbone of vertebrates, which is why they are in the same phylum, Chordata, that we are in.