Sponge
Like cnidarians (jellyfish, etc.) and ctenophores (comb jellies), and unlike all other known metazoans, sponges' bodies consist of a non-living jelly-like mass (mesoglea) sandwiched between two main layers of cells. Cnidarians and ctenophores have simple nervous systems, and their cell layers are bound by internal connections and by being mounted on a basement membrane (thin fibrous mat, also known as "basal lamina"). Sponges have no nervous systems, their middle jelly-like layers have large and varied populations of cells, and some types of cells in their outer layers may move into the middle layer and change their functions.
Sleep in non-human animals
Sleep as a phenomenon appears to have very old evolutionary roots. Unicellular organisms do not necessarily "sleep", although many of them have pronounced circadian rhythms. The jellyfish Cassiopea is the most primitive organism in which sleep-like states have been observed. The nematode "C. elegans" is another primitive organism that appears to require sleep. Here, a "lethargus" phase occurs in short periods preceding each moult, a fact which may indicate that sleep primitively is connected to developmental processes. Raizen "et al."'s results furthermore suggest that sleep is necessary for changes in the neural system.