The correct answer is option (A) ? "Acrania"
Explanation:
Acrania does not have a skull and it belongs to?sub-phylum Acraniata?which is also known as "Prochordata". The forerunners of the Vertebrata are Prochordates.
These Prochordata are called as Acrania due to the?absence of cranium?in them. They are classified on the basis of the notochord's nature. Further, they are classified into three sub-phyla commonly known as Cephalochordata, urochordate, and Hemichordata.
Acrania is a disorder in human being win which a full or partial ossein is absent in?cranial vault.
