1. Head
   
   1. Beaks: Descriptions can be found here: Lower beak; upper beak.
      
      
2. Photophores
   1. Large, compound photophores with posteriorly directed reflectors.
      

      Figure. Anteroventral mantle photophores of H. reversa, 29 mm ML, central North Atlantic, R/V G.O. SARS, MarEco cruise. Arrow points to the posterior reflector of one of the photophores. Photograph by R. Young.

   2. Arms IV with 4 longitudinal series of photophores on arm base (3 series of large photophores, dorsal series of small photophores
      

      Figure. Ventral view of the base of arm IV of H. reversa, 29 mm ML, central North Atlantic, R/V G.O. SARS, MarEco cruise. Arrows point to the dorsal series of small photophores. Photograph by R. Young.

   3. Terminal group of large photophores on arms absent.

Comments

More details of the description can be found here.

Species of the reversa-group are distinguished by the following characters:

1. Compound photophores
   1. Large and small photophores intermixed on ventral surface of mantle .
   2. 18 photophores (17 large and 1 small) around right eyelid.
2. Beak
   1. Weakly developed median ridge on each lateral wall.
3. Tubercles
   1. Absent.

H. reversa is most easily separated from other members of the reversa group by (1) the arrangement of photophores on arms IV (H. atlantica - 4 series, dorsal series of mixed sizes; H reversa - 4 series, dorsal series of small sizes; H. eltaninae - 3 series) and (2) from H. atlantica by the absence of simple, arm-tip photophores and from H. eltaninae by the short connective complex in their spermatophores and the presence of posterior reflectors (not described for H. eltaninae) on compound photophores.

The above information is from Voss (1969) and Voss, et al. (1998).