5. Oxygen comes from the
environment and carbon
dioxide in the end returns to
the environment. How do
small animals solve the
problem of taking away and
bringing these molecules
from / to their cells? Why isn't
that solution possible for
larger animals?

Gap-fill exercise

Fill in all the gaps, then press "Check" to check your answers. Use the "Hint" button to get a free letter if an answer is giving you trouble. You can also click on the "[?]" button to get a clue. Note that you will lose points if you ask for hints or clues!
Small animals whose make
direct contactor very close to
environment, like cnidarians
poriferans, make gas by
diffusion.

Larger with cells without
contact with the or far
from need special gas
systems. In these the
respiratory and circulatory systems
play role.