Protochordates are simple, marine animals that are part of the phylum Chordata. They are considered the closest relatives to vertebrates, sharing key features such as a notochord, dorsal nerve cord, and pharyngeal slits during some stage of their life cycle. Protochordates include groups like tunicates and lancelets, which exhibit these characteristics in their larval or adult forms. These organisms play a crucial role in marine ecosystems, often serving as filter feeders. They help maintain water quality by filtering out plankton and organic particles. Studying protochordates provides insights into the evolution of more complex animals, including fish and other vertebrates.