Small animals problem

Question

The oxygen comes from the atmosphere and carbon-dioxide in the end returns to the atmosphere. How do small animals resolve the problem of taking away and bringing such molecules from/to their cells? Why that isn’t answered possible for bigger animals?

Answer 1

The small animals whose tissues form direct contact or are much close to the atmosphere, such as poriferans and cnidarians, make gas exchange by the diffusion.

Bigger animals with cells devoid of direct contact with the atmosphere or far from it requirement special gas transportation systems. In such animals the respiratory and circulatory systems play this role.