An explanation that makes reference to three of the following points:
- oxygen \{dissolved / present\} in the water (1)
- large surface area (to volume ratio) for fast \{gas exchange / diffusion (of gases)\} (1)
- two \{layers (of cells) / cells thick\} so \{short diffusion distance (for gases) / fast gas exchange\} (1)
- \{tentacles / body\} move the water around to maintain (oxygen) concentration gradient (1)
IGNORE efficient and easy throughout NB penalise refs to \{lungs / alveoli / blood / vessels / etc\} within an awardable mp 2, 3 or 4 once
ACCEPT more diffusion IGNORE hollow body / tentacles
ACCEPT thin layer of cells so .....
ACCEPT other terms which clearly refer to tentacles e.g. tails, legs, fingers
(3)