shell = ljuska, kao npr. eggshell = ljuska od jajeta
Dijatomeje ili silikatne alge imaju tvrdu silkatnu ljusku
http://schaechter.asmblog.org/schaechter/2011/05/hard-biolog...
Diatoms are busy creatures, accounting by their photosynthesis for about 20% of the global primary production of organic material, an amount comparable to that produced by the tropical rain forests. They are single cells contained within a shell made up of glass (silica), which is what gives many of them often stunning shapes.

http://tolweb.org/Diatoms/21810
Basically, diatoms live in glass boxes. The silica shell of the diatom is called the 'frustule' and is made of two halves, each in turn composed of several different pieces. Hydrosera frustules, like those of all other diatoms, are perforated by many small holes, which allow water, dissolved material and solids (gases, inorganic nutrients, and organic substrates and secretions) to pass in or out.