A class of organisms composed of one or more cells containing a membrane-enclosed nucleus and organelles. This class constitutes one of three distinct evolutionary lineages (eubacteria, eukaryotes & archaea) of modern-day organisms. Includes all organisms except viruses and prokaryotes (mostly bacteria).
Organisms used to closely study and elucidate our understanding of basic biological phenomena that might not be easily researched in advanced organisms. In general, they possess characteristics such as small size, short life span and ease of cultivation.