Little crumb
Two names for the same organism, both of them about being broken.
A diatom builds itself a shell out of glass. Not metaphorical glass - actual silica, pulled out of seawater and laid down at ordinary temperatures into structures with pores and ribs and lattices arranged so precisely that people spent the nineteenth century using them to test the quality of microscope lenses. We need a furnace to make glass. They manage it in cold water, in the dark half of the day, while also getting on with everything else.
The name for that shell is a frustule.
Which comes from the Latin frustulum, a diminutive of frustum: a piece, a scrap, a morsel. A crumb.
Somebody looked at the most intricate small architecture in the natural world and called it a little bit of debris.
To be fair to whoever it was, they had a reason. Early microscopists were mostly not looking at living diatoms in seawater. They were looking at sediment, at dust, at scrapings and residues and the contents of other animals - and what turns up there is overwhelmingly wreckage. Empty shells, split halves, fragments. The glass outlasts the organism by an almost comical margin, which is why diatoms have such a good fossil record and why the first thing most people ever saw of one was the part left over after it died.
The same Latin root, incidentally, gives us frustum, the geometric solid you get when you cut the top off a cone. Also a piece of something. The word has always been about what remains after the cutting.
And then the organism’s own name is doing the same thing again.
Diatom comes from the Greek diatomos, cut through, from dia- (through) and temnein (to cut). It arrived via a genus named in the early 1800s and spread to the whole group.
That root is more familiar than it looks. Temnein is sitting inside anatomy, which is a cutting-up. Inside tome, which is a piece cut off a longer work. Inside epitome, a cutting-short. And inside atom - a-tomos, the thing that cannot be cut - which makes atom and diatom siblings with opposite meanings. One is what you cannot divide. The other is what is divided.
The name is a description of the machinery. A frustule is two halves, one fitting over the other like a petri dish and its lid. When the cell divides, each daughter keeps one half and builds a new one to fit inside it - which means the daughter that inherited the smaller half ends up slightly smaller than its parent. Do that for enough generations and the average cell in the population shrinks. They can only reset it by reproducing sexually and starting again at full size.
So the whole lineage is quietly running down in size, generation by generation, because of a consequence of the way the box is built. And the name they were given, before anyone understood any of that, means cut in two.
Two names for the same organism. One means crumb. The other means divided. Neither of them mentions the glass, or the lattice, or the fact that a decent fraction of the oxygen in the room came from something built this way.
Names are usually a record of what somebody noticed first. In this case what they noticed first was the breakage.