An organism may be either unicellular (a single cell) or, as in the case of humans, comprise many trillions of cells grouped into specialized tissues and organs. The term multicellular (many cells) describes any organism made up of more than one cell.
All organisms living on Earth are divided into the eukaryotes and prokaryotes based on the presence or absence of true nuclei in their cells. The prokaryotes represent two separate domains, the Bacteria and Archaea. Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus, and contain additional membrane-bound compartmentalization called organelles (such as mitochondria in animals and plastids in plants, both generally considered to be derived from endosymbiotic bacteria). Fungi, animals and plants are examples of kingdoms of organisms that are eukaryotes.
Daphnia are small, planktoniccrustaceans, between 0.2 and 5 mm in length. Daphnia are members of the orderCladocera, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style (although fleas are insects and thus only very distantly related). They live in various aquatic environments ranging from acidicswamps to freshwater lakes, ponds, streams and rivers.
The two most readily available species of daphnia are Daphnia pulex (small size & most common), and Daphnia magna (large). They are often associated with a related species from the order Cladocera known as Moina (In the Moinidae family instead of Daphniidae and much smaller than Daphnia pulex (approximately half the maximum length)). Daphnia eggs for sale are generally enclosed in ephippia (a thick shell, consisting of two chitinous plates, that encloses and protects the winter eggs of a cladoceran.)