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 th