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

$A$ true coelom is lined by which of the following germ layers?

The symmetry found in an animal which can be divided into two similar halves by one plane only is:

Radial symmetry is generally associated with ............ .

Assume that a pencil box held in your hand represents a plant cell. In how many possible planes can it be cut? Indicate these cuts with the help of line drawings.

Radial symmetry is often exhibited by animals having