Cephalochordata is a subphylum of marine animals known as lancelets. These small, fish-like creatures are characterized by their elongated bodies and notochord, a flexible rod that provides support. They possess a simple structure with a dorsal nerve cord and gill slits, which they use for filter feeding in shallow waters. Lancelets are considered important for understanding the evolution of chordates, as they share key features with more complex vertebrates. They inhabit sandy or muddy substrates and are typically found in coastal regions around the world. Their simple anatomy offers insights into the early development of vertebrate characteristics.