English: Morphology of Stygiellidae Living cells observed using Differential Interference Contrast (A–R); protargol-stained cells observed by bright-field microscopy (V–AG); TEM micrographs (S–U). Most cells presented here are grooved cells, only a few are swimming cells (J,M,R). Species and strains are arranged as follows: Stygiella adhaerens sp. nov. strain COORONG2 (A,B) and PETROCHORI (Z,AA); Stygiella incarcerata comb. nov. strain NORMAR (C,E,W), GOUVIA (D), and EVROS1I (V); Stygiella cryptica sp. nov. strain PC1 (F,X,Y); Stygiella agilis sp. nov. strain AND (G,J,AC), IGO3 (H,I), and MANG (AB); Velundella trypanoides gen. et sp. nov strain LUC3N (K,L,S–U,AD,AE) and BUSSPRAND (M); Velundella nauta sp. nov. strain BMAND (N–R,AF,AG). Bar = 5 μm (A–R,V–AG), 500 nm (S), and 200 nm (T,U). Labels – F1, axoneme of the posterior flagellum possessing single dorsal vane; F2, axoneme of the anterior flagellum; M, mitochondrion-related organelle without cristae; N, nucleus; Nu, nucleolus; arrow, anterior flagellum attached to the substrate; double-arrow, cytoplasmic projections or pseudopodia on a cell posterior of Stygiellidae or a bulbous protrusion on cell posterior of V. nauta sp. nov.