Chloroplast
In some algae (such as the heterokonts), chloroplasts seem to have evolved through a secondary event of endosymbiosis, in which a eukaryotic cell engulfed a second eukaryotic cell containing chloroplasts. This formed chloroplasts with three or four membrane layers. In some cases, such secondary endosymbionts may have themselves been engulfed by still other eukaryotes, so forming tertiary endosymbionts. In the alga Chlorella, there is only one chloroplast, which is bell-shaped.