Bulla, or BUBBLE-SHELL, a genus of Gasteropod Molluscs, formerly somewhat ambiguous, but now defined as one of the Tectibranchs or 'snails' with concealed posterior gills. In the family (Bullidæ) to which the bubble-shell belongs, the shell is thin, often almost globular, with concealed spire and wide aperture. It serves to protect the gills, and less frequently the entire animal. In some cases it is quite covered by the mantle and foot lobes. In the closely allied Haminea, the shell is horny and elastic; in the Sea-hares (Aplysia) it is very rudimentary; in some of the likewise nearly related Philinidæ it is entirely absent. The Bullidæ are carnivorous, and liny plates in the gizzard aid in internal mastication. There are numerous fossil and living species, all marine, and some British. See GASTEROPODS.