Acrania vs. Craniata — What's the Difference?

Acrania is a rare congenital disorder that occurs in the human fetus in which the flat bones in the cranial vault are either completely or partially absent. The cerebral hemispheres develop completely but abnormally.

Fishes; amphibians; reptiles; birds; mammals

(physiology) Partial or total absence of the skull.

Partial or total absence of the skull.

The lowest group of Vertebrata, including the amphioxus, in which no skull exists.