Hi Vicky, Bubble shells belong to a group of mollusks known as Opisthobranchs. Most of them do not have a shell, but some, like the bubble shells, do. These animals are hermaphroditic (both sexes in the same individual - self-fertilizing), and of those which have shells, most are ovoviviparous (the fertilized eggs are retained in the parent's body until they hatch). So, the tiny shells you found inside the bubble shells are embryonic, or newly hatched shells of the same species. The same can be found in some fresh water snails such as the genus Viviparus. (I am making one assumption here - that your reference to "cleaning bubble shells" means removing the soft body parts from live-collected bubbles. If you are talking about some kind of tiny shells that you found inside of dead, empty bubble shells, then there are many possibilities.). Paul M.