Birotules are dumbell shaped microscleres having 2 wheel shaped ends called roltules connected by a more or less cylindrical shaft.
Megascleres are large spicules 60–>200 micrometres (μm) in length and often function as the main
support elements in the skeleton. These can be spiny or smooth.
Microscleres are small spicules measuring 10–60 μm in length, scattered throughout the tissue and
are not part of the main support element. These can be spiny or smooth, needle shaped, rod shaped,
star shaped (asters) or dumb-bell shaped (birotules: having two wheel-shaped ends called rotules
connected by a more or less cylindrical shaft).
Gemmules are internal buds of sponges, usually <0.5 μm in diameter, which are the result of asexual
reproduction; these are formed by sponges for overwintering and can remain dormant through the
winter. Gemmules usually contain gemmoscleres and their arrangement can be used to identify
species, although some species can lack gemmoscleres. Other diagnostic characteristics include the
presence or absence of a tube surrounding the foraminal opening. (modified from Cocchiglia, et al., 2013)