Ctenophores are the comb jellies, and they superficially bear a strong resemblance to jellyfish. However, once you look past the body shape, they are completely different. On the body, ctenophores have 8 rows of hair arranged symmetrically in 4 pairs, used to propel them while swimming. This is the name-giving comb. Ventrally, they have a mouth/anus that opens straight to the gastric cavity where digestion takes place. Unlike jellyfish though, ctenophores swim with the mouth facing forward. We will see why this is a critical difference to the cnidarians later.
Ctenophores fall into two groups, the tentacle-bearing Tentaculifera and non-tentacle-bearing Atentaculata. They’re all marine predators, engulfing prey with their body. However, the tentaculate ctenophores have a special weapon called the colloblast. This is a special cell type unique to the ctenophores, and is present on the tentacles. When disturbed, the cell will attach to a prey and act as glue, allowing easy capture.
Ctenophores do not have any sexual ritual. They are hermaphrodites, releasing their gametes straight into the water column. Development proceeds directly, with no larval stage. They have a so-called “cydippid larva”, but this is not a true larva, as it is identical to the adult form. See Pang & Martindale (2008b) for more on it.
To the surprise of everyone who hears about it, ctenophores have a fossil record, and not just from one locality. They’re known from the Stephen Formation, related to the Burgess Shale (Conway Morris & Collins, 1996), and from the Devonian Hunsrück Slates (Stanley & Stürmer, 1983).
In many older papers and textbooks, one can find hypotheses uniting ctenophores and cnidarians as sister groups, i.e. that they are very closely related and share a last common ancestor exclusive to them. In fact, Linnaeus had classified a Cnidaria + Ctenophora group, as the Radiata, based on their sharing of a radial symmetry.
Similarly, others hypothesised of a group called the Coelenterata, which has the Cnidaria and Ctenophora as sister groups. The reasoning behind it is that both ctenophores and cnidarians are diploblastic, but this is nonsense as diploblasty is a plesiomorphic sate inherited from the last common ancestor, not a defining characteristic.
The fact of the matter is that the superficial resemblance to jellyfish is just that: superficial. Ctenophoran anatomy differs widely from any cnidarian anatomy, including that of any jellyfish, and they don’t share any of the defining characteristics of the cnidarians. Most importantly, they have no cnidocytes, the hyper-specialised stinging cells unique to the cnidarians.
So, whenever you see a phylogeny with Ctenophora and Cnidaria nested together, it is highly likely that there is something wrong there. It’s pretty rare to find it nowadays though.
So if they have little relation to the cnidarians, where do the ctenophores belong on the tree of animals? They are currently thought to be the sister group to the Bilateria – the bulk of animal diversity. The grouping is called the Acrosomata, after the structure of their sperm.
Ctenophore and bilaterian sperm differs from that of cnidarians and sponges in that it has a very distinctive structure at the end called the acrosome, with a perforatorium beneath it, and these two structures help in allowing the sperm enter the egg (it can be argued that there might have been novelties that occurred to the egg’s structure as well, but that’s still conjecture for now). This unique sperm structure allows us to confidently place the ctenophores and bilaterians in one group.
There is another characteristic unique to bilaterians and ctenophores: true muscle cells. In cnidarians, the muscle cells are epithelial, part of the skin. In the Acrosomata, the muscle cells are more specialised, separated from the skin as true myocytes. This is what allows the ctenophores to be active swimmers, rather than mostly passive drifters like the cnidarians.
Conway Morris S & Collins DH. 1996. Middle Cambrian Ctenophores from the Stephen Formation, British Columbia, Canada. Phil. Trans. R. Soc. B 351, 279-308.
Pang K & Martindale MQ. 2008a. Ctenophores. Current Biology 18, R1119-R1120.
Pang K & Martindale MQ. 2008b. Comb Jellies (Ctenophora): A Model for Basal Metazoan Evolution and Development. Cold Spring Harbor Protocols 2008.
Stanley Jr. GD & Stürmer W. 1983. The first fossil ctenophore from the Lower Devonian of West Germany. Nature 303, 518-520.