Amphioxus

Amphioxus, also known as Lancelet, are found buried in the sand in shallow parts of temperate or tropical seas. They are an important object of study in zoology as they provide indications about the origins of the vertebrates. They serve as an intriguing comparison point for tracing how vertebrates have evolved and adapted. Even though Amphioxus split from vertebrates more than 520 million years ago, their genomes hold clues about evolution, particularly how vertebrates have employed old genes for new functions. They are regarded as similar to the archetypal vertebrate form.

They grow up to about 5 centimeters (2.0 in) long, reaching 7 centimeters (2.8 in) at the longest. They have a translucent, somewhat fish-like body, but without any paired fins or other limbs. A relatively poorly developed tail fin is present, but they are not especially good swimmers. In common with vertebrates, lancelets have a hollow nerve cord running along the back, pharyngeal slits and a tail that runs past the anus. Also like vertebrates, the muscles are arranged in blocks called myomeres.