Euglena are
unicellular organisms that dwell on the surface of fresh water. They have their own
phylum,
Euglenophyta, because they share both
plant and animal characteristics. Like
protozoa they are
ovoid with a
flagellum for movement, and have a
cell membrane rather than a
cell wall so they can absorb other
organisms as food. However, like
algae they have
chloroplasts and will happily live off of
solar power as long as the sun is present. Some texts refer to them as
algae, and euglena are often responsible for "
algal bloom" conditions in fresh water. Euglena vary in size from .25 to .025 millimeters, and thus may be studied with an ordinary
microscope.
Flagellation is mediated by a red-colored organelle called the eyespot, which allows the euglena to orient towards light. This isn't for any predator-prey types of relationship, rather it is simply to find where the sun is so photosynthesis can be carried out. Interestingly, the eyespot is half covered by a black pigmented area which is pointed forward with respect to the euglena's motion. Thus, the organism isn't reacting to how much light the eyespot receives, but rather to where the shadow cast by the black spot is strongest. This makes sense when you realize the eyespot is a modified chloroplast, and is better at reacting noticeably (i.e., producing dramatically less of a given chemical) when it receives no light.
When in a dark environment for some time, the euglena enters a state called the palmelloid stage of its development, where it shrinks into a sphere and discards its flagellum. Instead of using photosynthesis for energy, it absorbs bacteria and amoebae through its cell membrane to be digested with oxygen by its mitochondria. If conditions become favorable for photosynthesis, the euglena's chloroplasts will kick in and provide enough energy to change shape and grow back the flagellum.
You may have noticed that I just mentioned mitochondria and chloroplasts in the same paragraph. At some point during euglena's evolution endosymbiotic events introduced both kinds of organelle into the organism, and they were both viable enough to last. Some biologists even point to an ancient Euglenophyte as the common ancestor of plants and animals. This makes sense, as it would've been a very early eukaryote, with enough prokaryotic flexibility left to evolve into such wildly different end forms.