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The vast array of living and fossil organisms comprising all taxonomic groups above the primitive unicellular prokaryotic level typified by the bacteria. The Eukaryotae thus include all plants and animals, the unicellular protists, and the fungi.
All organisms in this group possess an organized nucleus (or nuclei) in their cells, with a surrounding nuclear envelope and paired deoxyribonucleic acid–containing chromosomes; and elaborate cytoplasmic organelles, such as mitochondria, Golgi bodies, lysosomes, peroxisomes, endoplasmic reticulum, microfilaments and microtubules in various arrays, and, in photosynthetic forms, plastids or chloroplasts. Characteristically, centrioles, cilia, or flagelia are also present, the latter locomotory organelles composed mainly of tubulin with microtubules exhibiting a universal 9 + 2 pattern. See Cilia and flagella, Prokaryotae
Organization at the organismal level ranges from solitary unicellular to colonial unicellular, mycelial, syncytial (coenocytic), and truly multicellular with extensive tissue differentiation. Modes of nutrition run the gamut: absorptive, ingestive, photoautotrophic, plus combinations of these three major kinds. Life cycles vary tremendously, with haploidy more characteristic of “lower” groups and diploidy of the “higher” taxa. Reproduction, similarly, includes both asexual and sexual methods. In the “higher” multicellular forms, true embryos develop from the diploid zygote stage, which has resulted from fusion of sperm and egg cells.
Aerobic metabolism is commonly exhibited, especially by aquatic and terrestrial forms; anaerobic mechanisms exist, however, for numerous species found in poorly oxygenated habitats, including various sites within bodies of host organisms.
Size of species range from 1 micrometer (certain protozoa and algae) to many meters (whales, trees), Habitats cover all possible ecological niches: aquatic, terrestrial, and aerial, for free-living forms (many of which are motile, with the major exception of the trophic stage of various plants); and in or on all kinds of hosts, for symbiotic or parasitic forms (internal habitats, including cells, tissues, organs, or various body cavities). Dormant stages include cysts, spores, and seeds; these are often involved in dispersion or propagation of the species. See Animal kingdom, Fungi, Plant kingdom, Protozoa