||Zoology 250 (2013)
(Ctenophora study images;
click on tree to see full cladogram)
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- 1) An odd phylum of ~100 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 3 features with Cnidaria which they resemble superficially:
- a) diploblastic organization with extensive, mostly non-cellular middle layer (mesoglea?)
- b) possess only a mouth (no true anus?; but have curious anal pores)
- c) 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) possess 8 vertical "comb rows" (= ctene rows); combs are formed from paddle-like groups of cilia; iridescent in bright light
- b) comb cilia are the largest cilia known in Metazoa; ctenophores are the largest animals (up to 2m long!) still able to swim using cilia
- c) two long, branching tentacles covered with colloblasts (unique, very adhesive cell type) extend from deep tentacle sheaths
- d) adults exhibit conspicuous biradial symmetry
- 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) 2013 by A. Richard Palmer. All rights reserved.
(revised Jan. 3, 2013)