What features of euglena green indicate that the border between animals and plants is rather arbitrary?

Euglena green is a unique living organism that has caused a lot of controversy among botanists and zoologists. The contradictions arose due to the fact that euglena green has chloroplasts inside its cells, and chloroplasts have plants. The controversy continued for quite some time until it was possible to prove that green euglena belongs to the kingdom of animals, not plants. In addition to euglena, there are also some animals that are capable of photosynthesis. For example – sea slug, which is impressive in size and characteristic green color.