After the summer rains, small ponds often form on the ground, and if the ponds last for many days, the pool water tends to turn green. If we take a drop of green water and put it under a microscope, we can see many small green particles. On closer inspection, these small particles turned out to be some single-celled organisms, and there was a long flagella at the front of the fat body, and by the swing of this flagella, they could swim happily in the water.
Does this single-celled organism belong to animals or to plants? This simple question is a big headache for taxonomists.
Zoologists call it an animal on the grounds that they don't have a cell wall structure and can move freely, which is clearly an animal. Although it has chloroplasts in its body, it can only mean that it is a relatively special animal and an autotrophic animal. Imagine who would think it was a plant without these chloroplasts: so zoologists gave it a "passport" to the animal kingdom and gave it an animal name, the eye worm.
Botanists believe that although the eye worm does not have the cell wall of ordinary plants, it has vacuoles and photosynthetic carriers that animal cells do not have. The chlorophyll a and b contained in the chromosome, as well as β-carotene, lutein, etc. are also similar to higher plants. Although plant cells usually do not have flagella, during the reproductive period, all algae plants
Germ cells, as well as fungi, mosses, ferns, and some gymnosperms such as iron trees, also have flagella and can swim in water. As for the fact that the eye worm has no cell wall, it may indicate that it is the most primitive plant, and the cell wall has not yet evolved. So, botanists issued it a plant passport" and gave it a plant name, naked algae.
Now, it is generally believed that organisms such as eye worms are the common ancestors of animals and plants.