Classification
The number of known living ctenophore species is uncertain, since many of those named and formally described have turned out to be identical to species known under other scientific names. Claudia Mills estimates that there about 100 to 150 valid species that are not duplicates, and that at least another 25, mostly deep-sea forms, have been recognized as distinct but not yet analyzed in enough detail to support a formal description and naming.
The traditional classification divides ctenophores into two classes, those with tentacles (Tentaculata) and those without (Nuda). The Nuda contains only one order (Beroida) and family (Beroidae), and two genera, Beroe (several species) and Neis (one species).
The Tentaculata are divided into the following eight orders:
- Cydippida, egg-shaped animals with long tentacles
- Lobata, with paired thick lobes
- Platyctenida, flattened animals that live on or near the sea-bed; most lack combs as adults, and use their pharynges as suckers to attach themselves to surfaces
- Ganeshida, with a pair of small lobes round the mouth, but an extended pharynx like that of platyctenids
- Cambojiida
- Cryptolobiferida
- Thalassocalycida, with short tentacles and a jellyfish-like "umbrella"
- Cestida, ribbon-shaped and the largest ctenophores
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