The Ctenophora ( //; singular ctenophore, // or //; from the Greek κτείς kteis 'comb' and φέρωpherō 'carry'; commonly known as comb jellies) are a phylum of animals that live in marine waters worldwide. Their most distinctive feature is the "combs", groups of cilia they use for swimming, and they are the largest animals that swim by means of cilia – adults of various species range from a few millimeters to 1.5 meters (59 in) in size. Like cnidarians, their bodies consist of a mass of jelly, with one layer of cells on the outside and another lining the internal cavity. In ctenophores, these layers are two cells deep, while those in cnidarians are only one cell deep. Ctenophores also resemble cnidarians in having a decentralized nerve net rather than a brain. Some authors combined ctenophores and cnidarians in one phylum, Coelenterata, as both groups rely on water flow through the body cavity for both digestion and respiration. Increasing awareness of the differences persuaded more recent authors to classify them in separate phyla.
Ctenophores form an animal phylum that is more complex than sponges, about as complex as cnidarians (jellyfish, sea anemones, etc.), and less complex thanbilaterians (which include almost all other animals). Unlike sponges, both ctenophores and cnidarians have: cells bound by inter-cell connections and carpet-likebasement membranes; muscles; nervous systems; and some have sensory organs. Ctenophores are distinguished from all other animals by having colloblasts that capture prey by squirting glue on them, although a few ctenophore species lack them.
Like sponges and cnidarians, ctenophores have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the mesoglea in cnidarians and ctenophores; more complex animals have three main cell layers and no intermediate jelly-like layer. Hence ctenophores and cnidarians have traditionally been labelled diploblastic, along with sponges. Both ctenophores and cnidarians have a type of muscle that, in more complex animals, arises from the middle cell layer, and as a result some recent text books classify ctenophores as triploblastic, while others still regard them as diploblastic.