FIG. 28.5 Circumventing limits imposed by diffusion. (a) Sponges can attain a large size because the many pores and canals in their bodies ensure that all cells are in close proximity to the environment. (b) Jellyfish also have thin layers of metabolically active tissue, but their familiar bell can be relatively thick because it is packed with metabolically inert molecules (the mesoglea, or “jelly”).