Common name of a member of the Eucarya, one of the three domains of living organisms that are classified according to rRNA sequence homologies; they are further characterised by cells containing nuclei and membranes composed primarily of diacylglycerol derivatives. The other two domains are the Eubacteria (the bacteria), which have no nuclei (i.e. prokaryotes) and also have diacylglycerol derivative membrane components, and the Archaea (archaebacteria), which are also prokaryotes; they have a membrane composed of isoprenyl glycerol diether and tetraether lipids. Kingdoms within the Eucarya include plants, animals and fungi; kingdoms of the Archae are euryarchaeotes (or euryotes), which include methanogenic, halophilic, sulphur-reducing and some thermophilic organisms, and the crenarchaeotes (or crenotes), which include other sulphur-dependent organisms, thermoacidophiles and extreme thermophiles (eocytes).