Lancelet

The lancelet is a small, translucent, fish-shaped animal, typically 2.5 to 8 centimeters long, that burrows into soft seafloor sediment with only its front end exposed, filtering plankton from the water with a ciliated feeding current. It lacks a backbone, paired fins, or a well-developed tail, but it does carry a stiff, collagen-reinforced notochord running the length of its body and into its head, the feature that gives its group, the cephalochordates, their name. Because it shares so many organ systems with early vertebrates in simplified form, it is one of biology's key living models for understanding how the vertebrate body plan first evolved.