Complex sentence: Each cell of the sponge's body is therefore exposed to a constant flow of fresh oxygenated water, so can rely on diffusion across its cell membrane to carry out the gas exchange needed for respiration.
Split into simpler sentences: Each cell of the sponge's body is therefore exposed to a constant flow of fresh oxygenated water. They can therefore rely on diffusion across their cell membranes to carry out the gas exchange needed for respiration.