Why are lancelets sometimes called acraniates?

Lancelets are referred to as acraniates because they lack a braincase, or cranium, which is a defining characteristic of vertebrates.1

While vertebrates possess a well-developed head protected by a skull, lancelets—which belong to the subphylum Cephalochordata—have a poorly developed head and no such bony or cartilaginous enclosure for the brain. Consequently, the term "acraniate" (meaning "without a cranium") is used to distinguish them from the subphylum Vertebrata, which is also known as Craniata.12