Question

The animal without skull is
A) Acrania
B) Acephalia
C) Apteria
D) Acoelomatea

Answer 1

Hint: Sea squirts (Urochordata) is an organism which does not have the cranium. The chordate characteristic is lost in this kind of adult animal. They basically attach themselves to a very hard surface, thereby they tend to lose their tail, the nervous system, and these animals are not able to move.

Complete answer:
Acrania does not have a full skull and it basically belongs to the subphylum Acraniata, which is also well known as Prochordata.

Acraniata is defined as the group of organisms that do not have a distinct cranium, that is they have no true skull. They are in small and headless forms. They mainly include urochordates, cephalochordates, and hemichordates. These are cranium-free animals, which are present without jaws, vertebral columns, and paired appendages. There is Notochord but they are very less developed as compared to the craniates (organisms having a cranium).

Acrania in the human fetus is the condition where the flat bone which is usually present in the cranial vault is partially or completely absent. Acrania in humans takes place due to genetic disorders and it can easily be detected with the help of ultrasound during pregnancy. Acrania is the disorder present in human beings in which a full or a partial ossein is absent in the cranial vault.

Thus, the animal without a skull is Acrania. So, the correct answer is an option (A).

Note: These Prochordata are well known as Acrania because of the absence of cranium in them. They are classified into various parts on the basis of the notochord's nature. Further, they are again classified into three subphyla which are known as Cephalochordata, Urochordata, and Hemichordata.