Xennellidae. Cuticle annulated or smooth; longitudinal ridges on cuticle present or absent. Tapering cephalic capsule offset from rest of body by constriction, cuticular discontinuity and/or thickened cuticle. Female reproductive system with reflexed anterior ovary and rudiment of posterior genital branch. Male reproductive system monorchic (at least in X. suecica Allgén, 1935). Spicules short, arcuate; gubernaculum present or absent, precloacal supplements present or absent. Four species.
Xennella cephalata Cobb, 1920.
X. filicaudata Allgén, 1954.
X. metallica Tchesunov, 1988.
X. suecica Allgén, 1935.
In the classification of Filipjev (1925; 1934), Xennella was placed together with the genus Tycnodora Cobb, 1920, which has since been synonymized with Halalaimus de Man, 1888 by Lorenzen (1981), and Schistodera Cobb, 1920, which has since been synonymized with Oxystomina Filipjev, 1918 (family Oxystominidae Chitwood, 1935) by Hope & Murphy (1972). This placement reflected the apparently smooth cuticle of X. cephalata, although the cuticle of X. suecica and X. metallica is clearly annulated. De Coninck (1937) later provided a detailed description of the males of X. suecica, and indicated close similarities between Xennella and Dasynemoides Chitwood, 1936 based on the annulated cuticle with longitudinal ridges, structure of the cephalic capsule and arrangement of anterior sensilla.
The changing classification of Xennella partly stems from uncertainty regarding the structure of the amphids. While Cobb (1920) shows a pocket-shaped amphideal fovea in his original description of X. cephalata (which indicates relationships with the Enoplida), De Coninck (1965) shows a rounded amphideal fovea in X. suecica (which, together with other features, could indicate relationships with either the Desmodoridae Filipjev, 1922, Ceramonematidae Cobb, 1933 or Monoposthiidae Filipjev, 1934). Other authors only show an almond-shaped amphideal aperture without showing the structure of the amphideal fovea (Allgén 1935; Tchesunov 1988), which may have been obscured by the thick cuticle of the cephalic capsule (De Coninck 1937).