The mesoglea is a layer of jelly-like material between the external (ectoderm) and internal (endoderm) cellular layers of the body of a sponge or cnidarian. It consists largely of collagen-like fibers and mucopolysaccharides. In sponges it contains many motile cells. In cnidarians it may be thin and contain prolongations of cells of the body layers, as in Hydra, or bulky and tough containing many collagen fibers and many cells, as in sea anemones or large jellyfish. Mesoglea is the "jelly" in jellyfish!