Definition: (amended from Van Soest 2002:575): Poecilosclerida with smooth or acanthose ectosomal tornotes (basically diactinal, oxeote, but may be anisotornotes or styles), a peripheral choanosomal plumose skeleton of bundles or tracts of smooth or acanthose tornotes, a basal skeleton of acanthostyles, singly or in plumose bundles (may be absent), echinating acanthostyles (may be absent) or, a choanosmal confused arrangement of all spicule categories. Additional non-echinating acanthostyles or acanthoxeas may occasionally be present. Microscleres include arcuate chelas and sigmas, but may be absent. Ectosomal surfaces often but, not always with areolated porefields.
Discussion: To accommodate the new genus in Hymedesmiidae the definition (Van Soest 2002, p. 575) requires expanding to allow for the possession of acanthose ectosomal tornotes (other hymedesmiids have only smooth tornotes) and a confusedly plumose choanosomal arrangement of spicules which include the choanosomal tornotes. In addition, the ectosome is often but not always with areolated pore-fields. At present the family Hymedesmiidae contains 10 genera, none with acanthose tornotes or with a more confusedly plumose choanosomal arrangement of spicules. We present additional differences between Acantorna n. gen. and the existing genera of Hymedesmiidae (according to Van Soest 2002: 576-593):
- Acanthancora Topsent, 1927 has modified spined tuberculate chelae and a hymedesmioid arrangement of spicules.
- Hamigera Gray, 1867 has strongyles ot tylotes as tornotes, choanosomal smooth styles or subtylostyles.
- Hemimycale Burton, 1934 has no acanthostyles and no microscleres. Smooth strongyles and styles are not separated in ectosomal and choanosomal regions.
- Hymedesmia Bowerbank, 1864 are all thinly encrusting and have a hymedesmioid skeletal arrangement.
- Kirkpatrickia Topsent, 1912 has strongylote tornotes, lacks microscleres and “smooth or spined styles in plumose but anastomosing bundles.”
- Myxodoryx Burton, 1929 has strongylote tornotes, two categories of acanthostyles “in plumose but anastomosing bundles” and lacks microscleres.
- Phorbas Duchassaing & Michelotti, 1864 has “plumose tracts of tornotes and/or acanthostyles.”
- Plocamionida Topsent, 1927 has a “plocamiid basal skeleton. Megascleres mucronate tornotes, long acanthostyles, and basal acanthostrongyles.” Sigmas are absent.
- Pseudohalichondria Carter, 1886 has “plumose columns of smooth tornotes, without acanthostyles, with modified spined chelae.”
- Spanioplon Topsent, 1890 “has non-echinating accessory acanthostyles or acanthoxeas. No chelae.” Ectosomal anisotornotes are smooth.