Ctenophora
Taxonomy

Specific identification of ctenophores is a difficult task. Because of the problems outlined above, many original descriptions were based on small individuals (juveniles), often distorted or damaged by collection and preservation. As a result, descriptions of local races or varieties as new species are common. For example, at least 50 species were assigned to the genus Beroe, but most of them are synonyms (Tiffon, 1993). Care must also be taken with juveniles of several species in the class Tentaculata. Even adults without tentacles (Mnemiopsis maccradyi 4) pass through a "tentacular stage" during development, resembling Cydippida larvae (Mnemiopsis maccradyi 2), which further confuses proper identification.

In spite of the time elapsed since these earlier works, development of a classification system for Ctenophora has been very restricted. Efforts have been scarce due to the expensive equipment and methods required for an authoritative taxonomic revision of Ctenophora, particularly for pelagic and bathypelagic forms. In one submarine cruise, Harbison (1986) found over 22 mesopelagic species, of which only 5 had been described previously. Thus, it appears likely that most of the evolution within the group has taken place in the deep ocean, where high concentrations of ctenophores are found (Harbison et al., 1978) .

Because Ctenophora are a soft-bodied phylum and do not preserve in the fossil record, their phylogenetic relationships are still not well understood.

Harbison et al. (1978) summarized our present knowledge of the group; however, as the list of known species is continuously expanding, their classification is changing accordingly (Harbison and Madin, 1982; Harbison, 1985, 1996; Mills, 1998).