There are about 10,000 species of diatoms; these organisms make up a major portion of ocean plankton and are also a major source of food for many small marine animals. Unlike other members of the division, diatoms sometimes reproduce sexually via a process called syngamy. Diatoms are encased in double shells which are made of silicon and fit together like two halves of a pillbox; some of these microscopic boxes are quite beautiful. These organisms are sometimes able to move around on their own by secreting cilia-like fibrils along a special groove called the raphe. Diatom shells are used as abrasives in silver polish and toothpaste, as filters, and in highway reflectors.