In which of the following animals,respiration occurs without any respiratory organ?

Respiratory organs in aquatic arthropods like crayfish,prawn and molluscs like $Unio$ are

$O_{2}$ exchange with $CO_{2}$ by simple diffusion over the entire body surface takes place in:
$I.$ Sponges
$II.$ Coelenterates
$III.$ Flatworms
Select the correct option to complete the given statement.

The respiratory organ of insects is.......

With respect to the respiratory apparatus,pick out the incorrect pair/pairs.

The respiratory organs vary in different animals due to their