Foraminiferum

The third is a conglomerate of very minute tuff-grains, each enveloping a nucleus which proved on examination to be either a foraminiferum, a mollusk or, more rarely, a rounded particle of semi-transparent quartz.

Source: wiktionary

Mr. Hammond felt something of this practical education as he stood watching the dissolution of the grey chalk cliffs on the horizon, as the earthly kingdom, with its surging, struggling millions, its royalty and intellect, splendour and wisdom, had become but a faint outline on the narrow sea of a small quarter of a globe—itself only an orange in the universe. The whole concern merely a foraminiferum of creation.

Source: wiktionary

For of course, if this foraminiferum, living in a medium of fine chalk-grains, used those chalk-grains for the construction of its shell, it acquires a calcareous habitus, especially, after the diagnetic processes, it was subject to, during its fossilisation.

Source: wiktionary

Movements of intracellular particles along the lamellipodium formed by the foraminiferum Allogromia laticollaris.

Source: wiktionary