He says of the Spongi that they are moved by animalcula which inhabit them.
There is no doubt that animalcula of sorts get into these cavities and probably die there.
Animalcula is the plural of animalculum; there is no such word as animalculœ.
Animalculum (plural, animalcula) is used with the same meaning.
Perhaps the animalcula or infusoria furnish the best example.