Why was the genus Branchiostoma formerly called Amphioxus?

The genus Branchiostoma was formerly referred to as Amphioxus due to a naming conflict in early taxonomic history. The name Amphioxus was introduced in 1836 by the English naturalist William Yarrell, who chose it to describe the animal's shape, meaning "both ends pointed".1

However, the Italian zoologist Gabriele Costa had already named the same genus Branchiostoma two years earlier, in 1834. Under the established rules of biological taxonomy, the name published first takes priority. Consequently, Branchiostoma is the scientifically accepted name, while Amphioxus is now retained only as an informal term for these cephalochordate invertebrates.12