Answer
The organisms listed do not need respiratory organs since they are unicellular, with proportionally very large external surface area. This allows for sufficient gas exchange by simple diffusion to meet their needs.
Work Step by Step
From Fick's Law (p.1004), surface area is a critical feature for this exchange, and, when you consider these things (look at images of flatworms and ciliates in earlier chapters, as examples), you see that they already have lots of surface area across which diffusion can occur.
