di·a·tom 
(dīə-tŏm′)
(dīə-tŏm′)n.
Any of various microscopic one-celled or colonial heterokonts of the class Bacillariophyceae that are photosynthetic, have a silica cell wall made up of two interlocking parts, and form an important component of phytoplankton.
[New Latin diatoma, from Greek diatomos, cut in half, from diatemnein, to cut in half : dia-, dia- + temnein, to cut; see tem- in the Appendix of Indo-European roots.]
diatom