Family Acarnidae Dendy, 1922

Definition: Poecilosclerida with apically spined tylotes forming a tangential ectosomal skeleton (emended from Hooper, 2002; according Morrow & Cardenas, 2015).

Diagnosis: Encrusting, massive, flabellate or digitate growth forms, sometimes burrowing, many genera producing fistules. Ectosomal skeleton composed of tylotes, strongyles or modified tylote spicules bearing microspined bases forming tangential and/or paratangential tracts, often irregular or halichondroid in arrangement. Choanosomal megascleres are styles or modified styles (anisoxeas) forming reticulate structures in massive forms (isodictyal, isotropic, anisotropic or more irregular skeletons) or plumose or hymedesmioid skeletons in encrusting growth forms. Echinating spicules present or absent, consisting of acanthostyles, unique cladotylotes and tylostyles. Microscleres include palmate isochelae and toxas of various morphologies (both sometimes lost), with some genera also having other microscleres such as bipocillae, modified anisochelae, microrhabd-like spicules (modified microxeas or microstrongyles) and diamond-shaped microxeas that are considered modified toxas (emended from Hooper, 2002; our additions are in bold).