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 erratum Peña Cantero & Vervoort, 1997 ( Figs 4 , 16 D, 17D, 19D) Material examined. German Antarctic Expedition ANT-XXI/2: Stn PS65 /166-1, Polarstern , 15 December 2003 , 70°56.83’–70°56.00’, 10°32.61’– 10°30.53’W (off Cape Norvegia , Weddell Sea), 338– 253 m . Spanish Antarctic Expedition Bentart 95: Stn 24R, Hespérides , 30 January 1995 , 63°58.4715’S , 60°51.9882’W (off Trinity Island , Graham Land), 214 m (Department of Zoology, University of Valencia, BENTART 95-24R). Description. Monosiphonic (occasionally polysiphonic), unbranched stems, up to 130 mm high, usually not divided into internodes. Typically, perisarc strongly developed. Angle between cauline apophyses and stem ca. 70°. Cauline apophyses with two axillary nematophores, each provided with a much-reduced nematotheca ( Fig. 4 B). Hydrocladia branched, up to third-order hydrocladia reported. First hydrocladial internode bifurcated, with two similar prongs ( Fig. 4 A). Mesial-inferior nematophore on a strongly marked swelling at proximal third of internode ( Fig. 4 D–E); with scale-shaped nematotheca ( Fig. 4 D–F). Hydrotheca elongate, placed on middle of internode ( Fig. 4 A, D–E). Abcauline wall of hydrotheca basally straight, but strongly curving adcaudally at distal part ( Fig. 4 E). Hydrothecal aperture kidney-shaped ( Fig. 4 C), adcaudally directed ( Fig. 4 E). Gonothecae fusiform. Female gonotheca larger than male one, with subterminal, circular aperture. Male gonotheca with distal, circular aperture ( Fig. 4 G). FIGURE 3. Oswaldella elongata Peña Cantero, García Carrascosa & Vervoort, 1995 . A, hydrocladial branching and disposition of hydrothecae; B, cauline apophysis with axillary nematophores and ‘mamelons’; C, detail of cauline axillary nematophores; 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 4. Oswaldella erratum Peña Cantero & Vervoort, 1997 . A, hydrocladial branching and disposition of hydrothecae; B, cauline apophysis with axillary nematophores; C, unforked hydrocladial internode showing hydrothecal aperture and mesial superior and mesial inferior nematophores; D–E, unforked hydrocladial internodes (D frontal view, E lateral view); F, detail of mesial inferior nematotheca; G, gonotheca. Scale bar: 400 µm (A), 200 µm (F), 80 µm (D–E), 50 µm (B–C) and 20 µm (G). Remarks. The most remarkable finding in relation to this species is the presence of much-reduced nematothecae in the axillary nematophores, never described before.