||Zoology 250 (2017)
(Ctenophora study images;
click on tree to see full cladogram)
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- 1) An odd phylum of ~250 species; all marine, most pelagic; common members of the "gelatinous zooplankton"
- a) they can be significant predators in mid-ocean environment (eat fish larvae and zooplankton; e.g., Mnemiopsis invasion of Caspian Sea)
- b) most less than 5cm; body length can reach 2m! some have up to 70cm tentacles
- 2) Share 4 features with Cnidaria which they resemble superficially:
- a) a type of radial symmetry
- b) diploblastic organization with extensive, mostly non-cellular middle layer of extracellular matrix (mesoglea?)
- c) possess only a mouth (no true anus?; but have curious anal pores)
- d) gastrovascular cavity with radiating canals
- 3) Exhibit some curious differences from Cnidaria:
- a) asexual reproduction not well-developed
- b) all are hermaphroditic
- c) very peculiar, highly determinate, bi-radial cleavage
- d) lack cnidocytes
- 4) Four distinctive features:
- a) adults exhibit conspicuous biradial symmetry
- b) typically have 8 vertical 'comb rows' (= ctene rows); combs are formed from paddle-like groups of cilia; iridescent in bright light
- c) comb cilia are the largest cilia known in Metazoa; ctenophores are the largest animals (up to 2m long!) still able to swim using cilia
- d) two long, branched tentacles, covered with colloblasts (unique, very adhesive cell type), extend from deep tentacle sheaths
- 5) Evolutionarily enigmatic; some believe they are related to deuterostomes
- a) middle layer exhibits some tissue-like characters (contractile cells, nerve cells, mesenchyme cells); may not be true "mesoglea"
- b) middle layer develops from mesenchyme cells similar to primary mesenchyme cells in larval echinoderms
- c) wide variety of body forms: gooseberry (most common), lobate or ribbon-like (no tentacles); some benthic forms resemble flatworms)
- d) recent molecular data suggest they may be the most basal metazoan; stay tuned!
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Copyright (c) 2017 by A. Richard Palmer. All rights reserved.
(revised Dec. 19, 2016)