The number of living ctenophores is unknown due to the fact that identical organisms can actually be different species. Traditionally they are classified into two classes-- Tentaculata, those with tentacles, or Nuda, those without tentacles. Nuda has one order while Tentacula has eight in which variations on length and size of the organisms are specified.Ctenophores are small, multicellular marine animals that are similar in many ways to cnidarians. However, there are a few key differences:


Body Structure and Appearance:

Although phylum ctenophora is relatively small (it contains only about 90 members), there is still a high amount of variation with regards to body structure:
  • Ctenophores usually do not grow to be more than a few centimeters
    ctenophore cross-section. Source:
  • All ctenophores have radial or biradial symmetry.
  • Along with phylum cnidaria, ctenophores are diploblastic (Figure 1), meaning that their bodies consist of two germ layers, an outer layer, called the ectoderm, and the inner layer, called the gastroderm.Ctenophores have an oral and anal pore that facilitate feeding and excretion, respectively, as well as a stomach and system of internal canals that facilitate digestion. They have muscles, nervous systems, and some have sensory organs. The bodies of some ctenophores are tough enough to withstand tossing waves and sharp sediment particles, while others are very fragile, making it hard for scientists to collect and study them. With the exception of some deep sea species, like “Tortugas red,” (Figure 2) most ctenophores are transparent, although some have the ability to reflect blue, yellow and green light. Some species may also get their coloring from algae, with which they form symbiotic relationships.
    • Diploblastic.jpg
      Figure 1: Basic cross section of a dipoblastic organism
The mysterious, recently discovered, Tortugas Red
Video: Ctenophore bioluminescence

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