____ are characterised by their lack of well-defined nucleus and membrane bound organelles.
Bacteria
All living cells can be divided into two groups - prokaryotes and eukaryotes. Prokaryotes are characterised by their lack of well-defined nucleus and membrane bound organelles. All bacteria such as E. coli, Salmonella typhi, etc. are prokaryotes. Amoeba has a well-defined nucleus. Therefore, it is an unicellular, eukaryotic organism.