Diatoms are unicellular algae with a siliceous cell wall (frustule). They occur either as solitary cells or in colonies (shaped like ribbons, stars, fans). They contain chlorophyll for photosynthesis. A frustule is divided into two parts, the epitheca and the hypotheca. The epitheca overlaps the hypotheca, looking like the halves of a Petri dish. Valve view refers to the orientation of a frustule (the valve face is facing up). Girdle view means the girdle bands are facing up.