Complex sentence: Certain species of "Blepharisma" have served as model organisms for scientific research; and, since some varieties are easily cultured and readily available from scientific supply houses, they are a common object of study in school science classes.
Split into simpler sentences: Certain species of "Blepharisma" have served as model organisms for scientific research. Since some varieties are easily cultured and readily available from scientific supply houses, they are a common object of study in school science classes.