The Oct, described in Matter, are a crab-like species, described as a lower-level involved society, meaning that they are mentored by higher societies, though they in their specific case also mentor a non-involved society on the shellworld of Sursamen.
The Oct are described as having ovoid bodies the size of a human child's torso, coloured deep blue and covered with thick bristly green hairs. They have eight limbs (four of them serving as arms, four as legs), which are triple-jointed and described as 'broken-looking'. They are joined to the main body at black joint-stubs, which are set in rows along the side of the Oct's body in not-quite symmetrical order.
They are often seen with support backpacks which house anti-gravity equipment and support equipment which the Oct seem to need for survival or comfort, as the system is connected to 'face masks' and circulates fluids. They are also described as preferring to live underwater and often leaking or leaving behind fluids from their equipment or a thin fluid membrane which covers their bodies.
The Oct society is strongly centered around their belief (expressed in seemingly religious terms, though the Oct are known to be very incomprehensible) that they are the 'Inheritors' of the Veil, that species which long past created the shellworlds of the Culture universe before disappearing. However, other societies have shown this to be demonstrably false, and consider the Oct to be a little bit weird as well as somewhat pathetic. The Oct in turn have a strong hatred for the Aultridia, another species connected to the shellworlds.
Related: The Inheritors?,