Describe the key features of the Three-Domain System of classification.

(N/A) $\Rightarrow$ In the Three-Domain System,the Kingdom $Monera$ is divided into two domains. The remaining eukaryotic kingdoms constitute the third domain,which eventually leads to the Six-Kingdom classification.
$\Rightarrow$ In earlier classification systems,$Bacteria$,$Blue-Green Algae$,$Fungi$,$Mosses$,$Bryophytes$,$Pteridophytes$,$Gymnosperms$,and $Angiosperms$ were all included under the category of plants.
$\Rightarrow$ These organisms were grouped together because they all possessed a cell wall.
$\Rightarrow$ However,the organisms placed in this group differed widely in other characteristics. For example,$Bacteria$ and $Blue-Green Algae$ (prokaryotes) were grouped with eukaryotic organisms.
$\Rightarrow$ Furthermore,unicellular $Chlamydomonas$ and multicellular $Spirogyra$ were placed together under the group 'Algae'.
$\Rightarrow$ The system did not differentiate between heterotrophic $Fungi$ and autotrophic plants,despite differences in their cell wall composition (e.g.,$Fungi$ cell walls are made of $Chitin$,while plant cell walls are made of $Cellulose$).
$\Rightarrow$ Later,when these characteristics were considered,$Fungi$ were placed in a separate Kingdom $Fungi$.
$\Rightarrow$ All prokaryotic organisms were placed in the Kingdom $Monera$,and all unicellular eukaryotic organisms were placed in the Kingdom $Protista$.
$\Rightarrow$ In the $Protista$ kingdom,unicellular $Chlorella$ and $Chlamydomonas$ (which have cell walls) were placed together with $Amoeba$ and $Paramecium$ (which lack cell walls).
$\Rightarrow$ Modern classification systems focus not only on morphological,physiological,and reproductive similarities but are also based on phylogenetic relationships (evolutionary history).