Ctenophora (/tɪˈnɒfərə/; singular ctenophore, /ˈtɛnəfɔːr/ or /ˈtiːnəfɔːr/; from Ancient Greek: κτείς, romanized: kteis, lit. ‘comb’[7] and φέρω, pherō, ‘to carry’; commonly known as comb jellies) comprise a phylum of invertebrate animals that live in marine waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as “combs”), and they are the largest animals to swim with the help of cilia. Depending on the species, adult ctenophores range from a few millimeters to 1.5 m (4 ft 11 in) in size. Only 100 to 150 species have been validated, and possibly another 25 have not been fully described and named. The textbook examples are cydippids with egg-shaped bodies and a pair of retractable tentacles fringed with tentilla (“little tentacles”) that are covered with colloblasts, sticky cells that capture prey. Their bodies consist of a mass of jelly, with a layer two cells thick on the outside, and another lining the internal cavity. The phylum has a wide range of body forms, including the egg-shaped cydippids with retractable tentacles that capture prey, the flat generally combless platyctenids, and the large-mouthed beroids, which prey on other ctenophores.
Almost all ctenophores function as predators, taking prey ranging from microscopic larvae and rotifers to the adults of small crustaceans; the exceptions are juveniles of two species, which live as parasites on the salps on which adults of their species feed.
Despite their soft, gelatinous bodies, fossils thought to represent ctenophores appear in lagerstätten dating as far back as the early Cambrian, about 525 million years ago. The position of the ctenophores in the “tree of life” has long been debated in molecular phylogenetics studies. Biologists proposed that ctenophores constitute the second-earliest branching animal lineage, with sponges being the sister-group to all other multicellular animals.[8] Other biologists once believed that ctenophores were emerging earlier than the sponges, which themselves appeared before the split between cnidarians and bilaterians.[9][10] However reanalysis of the data showed that the computer algorithms used for analysis were misled by the presence of specific ctenophore genes that were markedly different from those of other species.[11][12] A recent molecular phylogenetics analysis concluded that the common ancestor of all modern ctenophores was cydippid-like, and that all the modern groups appeared relatively recently, probably after the Cretaceous–Paleogene extinction event 66 million years ago. Evidence accumulating since the 1980s indicates that the “cydippids” are not monophyletic, in other words, they do not include all and only the descendants of a single common ancestor, because all the other traditional ctenophore groups descend from various cydippids.[/vc_column_text][/vc_column][/vc_row]