I believe it’s because they had to now share their space with eachother
Protists are mostly unicellular (one-celled) eukaryotes. A few protists are multicellular (many-celled) and surprisingly large. For example, kelp is a multicellular protist that can grow to be over 100-meters long
Sort of it depends on your point of view of the story
All these sentences use the word "so" correctly except "Flies breathe through holes in
<span> their abdomen or thorax, so do bees.</span>" So should either begin a new sentence of be preceded by "and".