In diploblastic animals,what is present between the two germ layers?

Why is there a need for classification,and what are the bases of classification?

What type of body organization is shown in the given figure?

Heterotrophic,eukaryotic,multicellular organisms lacking a cell wall are included in the kingdom

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

Radial symmetry is often exhibited by animals having