Biol. [f. Gr. ζῴον animal + -OID: cf. late Gr. ζῳοειδής adj. resembling an animal.] Something that resembles an animal (but is not one in the strict or full sense): in early use applied somewhat widely, including, e.g., a free-moving animal or vegetable cell, as a spermatozoon or antherozooid; but chiefly restricted to an animal arising from another by asexual reproduction, i.e., budding (gemmation) or division (fission); spec. (and most usually) Each of the distinct beings or ‘persons’ which make up a compound or ‘colonial’ animal organism, and often have different forms and functions, thus more or less corresponding to the various organs in the higher animals. (Cf. ZOON.) Also attrib.
Often as the second element of a compound, as antherozooid, ascidiozooid, dactylozooid, gonozooid, siphonozooid, etc.: see these words.
1851. Huxley, in Ann. & Mag. Nat. Hist., Ser. II. VIII. 15. The term ‘zooid’ … is … intended to suggest … with regard to the creatures to which it is applied … that they are like individuals, and yet are not individuals, in the sense that one of the higher animals is an individual…. Instead of saying then, that in a given species, there is an alternation of so many generations, we should say that the individual consists of so many zooids. Ibid., 17. Zooid Development by External Gemmation, Internal Gemmation.
1855. W. S. Dallas, in Orr’s Circ. Sci., Org. Nat., II. 456. The individual Salpa consists of two zooids, one oviparous, the other gemmiparous…. In the Aphides, as many as eleven consecutive series of gemmiparous zooids have been observed to intervene between two periods of sexual reproduction.
1864. [see ZOON].
1870. Rolleston, Anim. Life, p. lxxxvi. In the Polyzoa polymorphic zooids are produced by gemmation.
1871. T. R. Jones, Anim. Kingd. (ed. 4), 98. In each colony [of hydroids] the alimentary and reproductive functions are respectively intrusted to two distinct kinds of zooids,… the nutritive and the sexual polypites…. The sexual zooids, like the flower-buds of plants, are only developed at certain seasons.
1888. Rolleston & Jackson, Anim. Life, 711. Rhabdopleura forms indefinitely branching colonies…. The zooids are all connected by a stem. Ibid., 745. The sexual zooid is developed from the asexual, either directly by metamorphosis, or indirectly by gemmation or fission, thus giving rise to an Alternation of Generations.
Hence Zooidal a., pertaining to a zooid or zooids; Zooidiogamous a., characterized by, or of the nature of, fertilization in which a ‘zooid’ or motile cell (e.g., an antherozooid) unites with another cell.
1886. Geol. Mag., Dec., 535. The larger [tubuli] I regard as zoöidal tubes.
1891. Nature, 17 Sept., 484/1. Karyogamy is … Zooidiogamous: one gamete at least is actively motile.
1907. Amer. Nat., June, 362. Ancient zoidogamous Gymnosperms.
