Depending on the species, adult ctenophores range from a few millimeters to 1.5 m (5 ft) in size. Only 100 to 150 species have been validated, and possibly another 25 have not been fully described and named.
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 a pair of 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 (Porifera Sister Hypothesis). Other biologists contend that ctenophores were emerging earlier than sponges (Ctenophora Sister Hypothesis), which themselves appeared before the split between cnidarians and bilaterians. Pisani et al. reanalyzed the data and suggested 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. Follow up analysis by Whelan et al. (2017) yielded further support for the Ctenophora Sister hypothesis, and the issue remains a matter of taxonomic dispute. Schultz et al. (2023) found irreversible changes in synteny in the sister of the Ctenophora, the Myriazoa, consisting of the rest of the animals.
^Conway Morris, S.; Collins, D. H. (29 March 1996). "Middle Cambrian Ctenophores from the Stephen Formation, British Columbia, Canada". Philosophical Transactions of the Royal Society B: Biological Sciences. 351 (1337): 279–308. Bibcode:1996RSPTB.351..279C. doi:10.1098/rstb.1996.0024.