Why are cephalochordates also called lancelets?

Cephalochordates are commonly called lancelets because of their streamlined, fishlike appearance. The name refers to their slender, blade-like shape, which tapers at both ends.123

While the term "cephalochordate" (derived from the Greek kephale, meaning "head") describes the anatomical feature where the notochord extends into the anterior of the body, "lancelet" serves as a descriptive common name for these small, translucent marine invertebrates. They are also frequently referred to by the informal name Amphioxus.431