[2] It has a global distribution, although several genera occur primarily in colder temperate waters, and several have very restricted ranges.

[3] The outer skeleton is made of tylotes (long, slender megascleres with knobs at both ends) which may be modified to have a spined base as well as strongyles (megascleres with both ends blunt or rounded).

These form massive networks to create isodictyal (triangular meshes in which each side is one spicule long), isotropic (no differentiation into primary or secondary tracts, lines or fibers), anisotropic (primary and secondary tracts, lines or fibers) or more irregular skeletons.

In encrusting forms they form plumose (skeletal structures radiate from primary fibres or spicule tracts) or hymedesmioid (monoaxon megascleres arranged singly with the heads fixed to a spongin base with the points directed outwards) skeletons instead.

When present, they take the form of acanthostyles (spiny styles) and/or unique cladotylotes (megascleres with 4-8 or even more axial branches and tubercles).