SEM study of species of Oswaldella Stechow, 1919 (Cnidaria, Hydrozoa, Kirchenpaueriidae), with an annotated checklist of the species of the genus Author Molinero, A. González Author Peña Cantero, A. L. text Zootaxa 2015 4052 4 401 441 journal article 10.11646/zootaxa.4052.4.1 93f6d085-8e63-4d1e-89c1-f950128b2a61 1175-5326 245887 47EF6E9D-9064-4899-B3DD-276FF7C969EB Oswaldella rigida Peña Cantero, Svoboda & Vervoort, 1997 ( Figs 11 , 16 K, 18C, 20C) Material examined. New Zealand Antarctic Expedition TAN0402: Stn 88 , Tangaroa , 14 February 2004 , 72°05'86''S , 172°55'81''E (Cape Adare, Ross Sea), 515 m . Description. Monosiphonic, usually unbranched stems, up to 210 mm high, usually divided into internodes. Angle between cauline apophyses and stem 70–90°. Perisarc strongly developed. Cauline apophyses with two axillary nematophores, each emerging through simple perisarc holes ( Fig. 11 B–C). Hydrocladia typically bifurcated ( Fig. 11 A), but up to third-order branching present. First hydrocladial internode bifurcated, with two similar prongs ( Fig. 11 B). Mesial inferior nematophore emerging from marked swelling at proximal third of internode ( Fig. 11 D); with strongly developed nematotheca ( Fig. 11 D, F). Hydrotheca elongate, roughly placed on middle of internode. Stony appearance. Abcauline wall straight, but convex at basal part. Hydrothecal aperture circular, roughly perpendicular to long axis of internode ( Fig. 11 D–E, G); rim typically uneven, with slight adcauline elevation. Gonothecae elongated with large subterminal aperture. Remarks. The most striking feature of O . rigida is the strong stony appearance under SEM. FIGURE 10. Oswaldella niobae Peña Cantero & Ramil, 2006 . A, stem fragment with hydrocladium showing hydrothecal arrangement; B, cauline apophyses showing two axillary nematophores and one ‘mamelon’; C, cauline apophysis with a single axillary nematophore; D–E, unforked hydrocladial internodes (D frontal view, E lateral view). Scale bar: 400 µm (A), 100 µm (D–E) and 20 µm (B–C). FIGURE 11. Oswaldella rigida Peña Cantero, Svoboda & Vervoort, 1997 . A, hydrocladial branching and disposition of hydrothecae; B, cauline apophysis with axillary nematophores and branched hydrocladium; C, detail of cauline apophysis showing axillary nematophores; D–E, unforked hydrocladial internodes (D lateral view, E frontal view); F, detail of mesial inferior nematotheca; G, detail of hydrothecal aperture and mesial superior nematophore. Scale bar: 800 µm (A), 100 µm (B, D– E) and 10 µm (C–F). We have observed two axillary nematophores on every cauline apophysis examined ( Fig. 11 B–C). Nevertheless, Peña Cantero et al . (1997) indicated that apophyses with a single axillary nematophore were also present. In this sense, it is worth mentioning that we have also observed a tendency to fusion of the two axillary nematophores.