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Summary.

  • 1
    The occurrence of a perioa of heavy mortality among the sea-water fish in the London Aquarium in 1929 and 1930 and its amociation with a parasitic infection is discussed.
  • 2
    The vegetative stage of the parasite which occurs in the gills and body-surface of the fish is described. It is shown to be a species of Dinoflagellate belonging to the genus Oodinium, as described in a preliminary note 1931 (3). The resemblances to, and differences from, other members of the genus are pointed out, and the mode of nutrition of the organism and the function of certain structures comparable with the pusule apparatus of free-living forms are discussed, and also the form of the organs of fixation. The organiem is regarded as a true parasite living at the expense of its host.
  • 3
    The behaviour of the parasite on being freed from its host is noted, and a description of its division up to the production of motile dinospores is given, together with a description of the dinospores themselves.
  • 4
    The conditions under which sporulation occurs are discussed, showing that temperatures from 23–27° C. are favourable, and that the process stops at low temperatures, the parasite remaining quiescent. Sporulation occurs at pH 8 and over.
  • 5
    The conditions under which the parasite occurs in the Aquarium are noted, temperature again being an important factor, and the pH being usually 7–2–7.6.
  • 6
    The animals infected and the origin of the parasite and the reasons for its success in the Aquarium are investigated. It is regarded as being imported from Bermuda and dying out when importation ceases. A similar parasite is also known on Amphiprion in Berlin and Amsterdam, where no fish are imported from Bermuda.
  • 7
    The symptoms and effoct of parasitic infection of the fish and cures which have been attempted are described. No satisfactory cure has been discovered.
  • 8
    The species O. ocellatum is redefined.