Arachnoidiscus ehrenbergi is a large centric diatom sometimes reaching almost a milimeter in diameter. In both the scanning electron micrograph (left) and in transmitted light micrograph (right), you can clearly see the strong radial ribs which distinguish the genus from other genera such as Coscinodiscus. Diatoms are a type of algae, a general catagory now recognized as containing several distinct taxonomic groupings. Diatoms live as single cells or in chains linked together. One distinguishing feature of the group is the protective covering, or frustule, made of silicon dioxide which the individual cells secrete by collecting silica ions out of the water. Diatoms live in both freshwater and in the oceans. Arachnoidiscus ehrenbergi is fairly common in the Pacific Ocean.