Individual cells of Volvox resemble the appearance of Chlamydomonas species. They are round or oval and exhibit a pair of flagella and a red eyespot, or stigma. The flagella of the organisms extend outside of the colony, which may contain as may as 50,000 individuals and can frequently be seen by the naked eye. The hollow, mucilaginous balls, which reach up to 1 millimeter in diameter, are often found spinning gracefully through the water of ponds, puddles, and ditches. In areas of high nitrogen content, Volvox blooms may occur, especially in the summer months. In fact, Volvox may be used as an indicator species, alerting ecologists of instances of nitrate enrichment.