Z

Zeitgeber Any entraining agent that plays a role in setting or resetting an internal biological clock. Examples include sunrise or sunset.

zero population growth (ZPG) The number of births at which people are just replacing themselves; also called the replacement level of fertility.

zoea an early larval stage of many crustaceans

zoecium , zooecium Cuticular sheath or shell of Ectoprocta.

zoid Member of a colonial organism.

zoitocyst Tissue phase in some of the coccidia of the Isospora group. They usually have internal septae and contain thousands of bradyzoites. Also called a sarcocyst or Miescher's tubule.

zonation The presence of organisms within a particular range, as in the vertical zonation observed in the intertidal.

zone of aeration Upper soil layers that hold both air and water.

zone of differentiation An area within the plant, proximal to the apical meristems, where cells begin to assume their srtuctural and functional characters.

zone of leaching The layer of soil just beneath the topsoil where water percolates, removing soluble nutrients that accumulate in the subsoil; may be very different in appearance and composition from the layers above and below it.

zone of saturation Lower soil layers where all spaces are filled with water.

zonite The individual body unit of a member of the phylum Kinorhyncha.309

zoochlorella Any of various minute green algae (usually Chlorella) that live symbiotically within the cytoplasm of some protozoa and other invertebrates.

zooflagellates Members of the Zoomastigophora, the animal-like flagellates (phylum Sarcomastigophora).

zooid An individual member of a colony of animals, such as colonial cnidarians and ectoprocts, produced by incomplete budding.

zoology The study of animals.

Zoomastigophorea The protozoan class where members lack chloroplasts; heterotrophic or saprozoic. Examples: Trypanosoma, Trichonympha, Trichomonas, Giardia.

zoonosis Disease of animals that is transmissible to humans. Some authors subdivide the concept into zooanthroponosis, an infection humans can acquire from animals, and anthropozoonosis, a disease of humans transmissible to other animals.

zooplankton The animal component of the plankton, having only limited locomotory powers.

zoospore A motile asexual spore.

zooxanthellae A group of dinoflagellates that live in mutualistic relationships with some cnidarians. They promote high rates of calcium carbonate deposition in coral reefs.

Zosterophyllophyta An extinct division of early vascular plants characterized by lateral sporangia and commonly spine-like emergences. Believed by evolutionary biologists to be the ancestor of the lycopods.

Zugunruhe Restlessness that an animal displays at the time of its usual migration.

Zygoma yoke.

zygomorphic flower A bilaterally symmetrical flower, capable of being divided into two symmetrical halves only by a single longitudinal plane passing through the axis.

Zygomycetes Members of the fungal division Zygomycota, consisting of about 800 terrestrial species having primarily coenocytic hyphae. Bread mold is an example.

zygosporangium (or zygospore)The thick-walled sexual spore formed by members of the zygomycetes.

zygospore A thick-walled, resistant spore resulting from a zygote.

zygote Diploid cell produced by the fusion of an egg and sperm; fertilized egg cell.

zygotic meiosis. Meiosis that takes place within the first few divisions after zygote formation; thus all stages in the life cycle other than the zygote are haploid.