Diatoms convert the products of photosynthesis to oils and fats, unlike plants which convert them to carbohydrates. Diatoms are perfectly symmetrical. The ones shown in the image are Pennates (Class: Pennales) because they are symmetrical left/right - perfectly. Although most of the Diatoms in the image are "boat shaped" ovals, I've circled a few that are rectanglar and "hour glass" shaped. There are approximately 10,000 species of Diatoms, and the fossil record shows that some lived more than 66 million years ago.