Archive image from page 603 of The depths of the ocean. The depths of the ocean : a general account of the modern science of oceanography based largely on the scientific researches of the Norwegian steamer Michael Sars in the North Atlantic depthsofoceangen00murr Year: 1912 568 DEPTHS OF THE OCEAN (4) An abyssal layer (1500 to 5000 metres), in which the large Challengeridse {Frotocystis fiaresi, P. thomsoni) are found. The multicellular animals are all represented among the pelagic forms, from the medusae to the fishes. Commencing with the Coelenterates we may mention the Medusae, the Siphono
Archive image from page 603 of The depths of the ocean. The depths of the ocean : a general account of the modern science of oceanography based largely on the scientific researches of the Norwegian steamer Michael Sars in the North Atlantic depthsofoceangen00murr Year: 1912 568 DEPTHS OF THE OCEAN (4) An abyssal layer (1500 to 5000 metres), in which the large Challengeridse {Frotocystis fiaresi, P. thomsoni) are found. The multicellular animals are all represented among the pelagic forms, from the medusae to the fishes. Commencing with the Coelenterates we may mention the Medusae, the Siphonophores, the Ctenophores, and the larval Actiniae. Medusre. The Medusae are generally bell-shaped or globular, with a more or less transparent jelly-like body. On the edge of the bell some forms have a band-shaped fold or moulding (' craspedon'), and accordingly the medusae are divided into two main groups: Craspedota with a craspedon, and Acraspeda without a craspedon. The Craspedota comprise four groups: Anthomedusae, Leptomedusae, Fig. 395. Tiiscaretta globosa (Borgert), subsp. chiini, Haecker (about '). (From Haecker.) Trachymedusae, and Narcomedusae, of which the first two are mero- pelagic and the last two holopelagic. The meropelagic forms pass through an ' alternation of generations,' the eggs produced by the medusae develop into larvae which attach themselves to the bottom and grow into hydroid polyps or zoophytes ; by ' budding' the zoophytes produce small medusae, which lead a swimming pelagic life and produce eggs. Fig. 396 shows a colony of hydroids with different stages of medusae developing, and Fig. 397 shows one of the medusae just after leaving the colony. The Craspedota are therefore termed hydroid medusae or hydromedusae, although they include two groups with no alternation of generation and no bottom stages, which are supposed to be descended from neritic forms. The hydromedusae having an alternation of generations are represented by a vast number of
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