What is "branchiostomidae"

Branchiostomidae (syn: Branchiostomatidae) is the family that includes most of the lancelets, which are tiny chordates without a true spine but only a flexible notochord.

Almost all lancelets belong to Branchiostoma, the one and only genus in the family Branchiostomidae. The genus was originally going to be called Amphioxus but was later changed to its current name.