Why are algae belonging to the Euglenoid family thought to be related to the protozoa?

Answer 1

Striaght from wiki

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Protozoa (in Greek proto = first and zoa = animal) are single-celled eukaryotes (organisms whose cells have nuclei) that show some characteristics usually associated with animals, most notably mobility and heterotrophy. They are often grouped in the kingdom Protista together with the plant-like algae and fungus-like water molds and slime molds. In some newer schemes, however, most algae are classified in the kingdoms Plantae and Chromista, and in such cases the remaining forms may be classified as a kingdom Protozoa. The name is misleading, since they are not animals.

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http://en.wikipedia.org/wiki/Euglena



The Euglenids are one example of why the Kingdom Protista (aka Protoctista) makes so much sense in comparison to the older taxonomy, in which protozoa were grouped with the animals and chloroplast containing algae were considered plants. In this group of single celled organisms, the Eugeloids, which are obviously derived from the same ancestral organism, we have both organisms containing chloroplasts that synthesize their own sugars using sunlight and dissolved carbondioxide - and organisms that do not have the capability to perform photosynthesis and thus must ingest or absorb their food.



The complex chloroplasts of Euglena (and those of photosynthetic dinoflagellates) are surrounded by three membranes. This is unlike chloroplasts of higher plants and most green algae, which are surrounded by two membranes. The additional membranes present a barrier to import of chloroplast precursor proteins not present in the simpler double membrane chloroplast envelope. In higher plants and most green algae, chloroplast proteins are synthesized on free polysomes and then posttranslationally imported directly into the chloroplast. Protein import into these complex chloroplasts is fundamentally different and occurs in three distinct steps: 1) Synthesis on polysomes bound to the rough endoplasmic reticulum (ER) with co-translocation into the ER; 2) Transport from the ER into the Golgi apparatus; and finally 3) Import across the three chloroplast membranes.
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Now in my own words. . . . you see how their chloroplasts are different from plants, the presence of flagella and their mode of consuming food. . . . so it's no wonder they are usually classified as protists.

You just need to read what I have pasted above to fully understand.