An intracellular non-membrane-bounded organelle; a hollow structure made of protein, which usually has the form of a cylindrical tube closed by conical end caps. By regulating their relative gas vesicle content, aquatic microbes are able to perform vertical migrations.
Comment:
Note that although this organelle is commonly referred to as a 'vesicle' or 'vacuole' in the literature, it is not surrounded by a membrane.