Cephalochordate

Model organism

Lancelets became famous in the 1860s when Ernst Haeckel began promoting them as a model for the ancestor of all vertebrates. By 1900, lancelets had become a model organism. By the mid-20th century they had fallen out of favor for a variety of reasons, including a decline of comparative anatomy and embryology, and due to the belief that lancelets were more derived than they appeared, e.g., the profound asymmetry in the larval stage. More recently, the fundamental symmetric and twisted development of vertebrates is the topic of the axial twist theory. According to this theory, there is a deep agreement between the vertebrates and cephalochordates, and even all chordates.