Which animal has unpaired lungs?

Which of the following respires by gills?

Select the correct option regarding the respiratory organs in different animals:

$O_{2}$ 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.

Which of the following is not a character of a respiratory surface?

Name the organs of respiration in the following organisms:
$(a)$ Flatworm
$(b)$ Birds
$(c)$ Frog
$(d)$ Cockroach