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

In the given chart,what do $a$ and $b$ represent respectively?
$(i)$ Acoelomate
$(ii)$ Hemichordata
$(iii)$ Agnatha
$(iv)$ Pseudocoelomate
$(v)$ Gnathostomata
$(vi)$ Cephalochordata

Coelom derived from blastocoel is known as

What is it called when any plane passing through the central axis of the body divides the organism into more than two identical halves?

In the given figure,identify the label marked $(a):-$

What is the difference between direct and indirect development?