The main basis of classification of the phylum $Protozoa$ is:

When any plane passing through the central axis of the body divides the organism into two identical halves,the organism is called $-$

Animals are classified on the basis of which of the following features?
$I$. Coelomic cavity
$II$. Level of organisation
$III$. Notochord
$IV$. Skeletal structure

What does '$a$' indicate in the given figure?

What are animals possessing a coelom called?

In which animals is the germinal layer organization shown in figure $B$ observed?