In contrast with Kingdoms Bacteria and Archaea, whose members have prokaryotic cells, all members of Kingdom Protista have eukaryotic cells. The organisms comprising this kingdom are very diverse and heterogeneous, but none have the distinctive combinations of characteristics possessed by members of Kingdoms Plantae, Fungi, or Animalia. Many, including the euglenoids, protozoans, and
some algae, consist of a single cell, whereas other algae are multicellular or occur as colonies or filaments. Nutrition is equally varied, with the algae being photosynthetic, the slime molds and protozoans ingesting their food, the euglenoids either carrying on photosynthesis or ingesting their food, and the oomycetes and chytrids absorbing their food in solution.
Individual life cycles vary considerably, but reproduction is generally by cell division and sexual processes. Many protists (mostly single-celled members of Kingdom Protista) are motile, usually by means of flagella, but other members of the kingdom, especially those that are multicel-lular, are nonmotile, although most of the multicellular members produce some motile cells.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.