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A new species of the genus Metarhabditis, M. giennensis sp. n., collected from a riverbank poplar forest in the southern Iberian Peninsula, is described and illustrated, including SEM observations. The new species is characterized by its 1.01-1.16 mm long body in females and 0.77-0.98 in males, cuticle with very fine transverse striation, lip region 9-14 µm broad and consisting of six swollen rounded lips fused in pairs, stoma 14-25 µm long with tubular gymno-promesostegostom, pharynx with slightly swollen metacorpus and slender isthmus, nerve ring, excretory pore and deirids located at isthmus level, female reproductive system didelphic-amphidelphic, vulva equatorial (V = 49-50), female rectum length 1.2-1.7 times the anal body width, female tail conical-elongate with acute tip (123-199 µm, c = 5.8-8.2, c' = 8.2-11.1), male tail conical (34-56 µm, c = 15.5-25.7, c' = 2.4-3.5), bursa leptoderan, 32-41 µm long spicules with rounded and ventrally bent manubrium, and 17-20 µm long gubernaculum. An updated list of species of the genus and a key for their identification are also provided. [Zoobank URL: urn:lsid:zoobank.org:act:7EBAC9DB-A148-4A1A-97E5-3B796D0839BF].

Three species of the genus Sectonema, namely S. demani, S. heynsi and S. macrospiculum, originally described by E. Altherr, are re-described on the basis of type material. Detailed descriptions, morphometrics and illustrations, including line drawings and/or LM pictures, diagnoses and remarks about their identity and later reports are provided for each species. Sectonema demani is characterized by its 6.78-7.24 mm long body, lip region 27-28 µm broad, mural tooth 19-20 µm long at its ventral side, pharyngeal expansion 834-875 µm long or 68-69% of total neck length, uterus a simple tube-like structure 2.1-2.2 times the corresponding body diameter, V = 49-52, tail short and rounded conoid (51-55 µm, c = 123-143, c' = 0.7-0.8), and male unknown. Sectonema heynsi is distinguished by its body 6.45 mm long, lip region 28 μm broad, circumoral area bearing cilia- or seta-like structures, mural tooth-like structure 20 μm long, pharyngeal expansion 856 μm long or 68% of total neck length, uterus a tube-like structure ?1.7 times the corresponding body diameter, V = 50, tail short and rounded to conoid (54 μm, c = 120, c' = 0.7), and male unknown. And S. macrospiculum is characterized by its 4.07-5.06 mm long body, lip region 27-28 μm broad, ventral side of the odontostyle 15 μm long, pharyngeal expansion 555-640 μm long or 67-68% of total neck length, uterus a simple tube-like structure 1.9-2.1 times the corresponding body diameter, V = 53, female tail short and convex conoid with rounded terminus (62 μm, c = 82, c' = 0.7), male tail more conoid than that of female and with the ventral side visibly more straight (58 μm, c = 71, c' = 0.8), spicules 107 μm long, and eight irregularly spaced ventromedian supplements bearing hiatus.

Three new and one known species of the genus Aporcelaimellus, collected mainly in natural areas from the Iberian Peninsula and characterized by having a simple uterus and atypical caudal region are described and illustrated, including line drawings, LM pictures and/or SEM pictures. Aporcelaimellus deserticola sp. n. is distinguished by its body 1.79-2.43 mm long, lip region 14-16 pm broad and offset by expansion rather than by constriction, odontostyle 18-21 µm long, neck 464-555 µm long, pharyngeal expansion 234-283 µm long or 49-52% of total neck length, a dorsal cell mass present at level of anterior end of intestine, uterus 34-67 µm long or 0.6-1.1 times the corresponding body diameter, V= 49-55, tail conical with rounded terminus (29-43 µm, c = 47-66, c' = 0.9-1.4) and bearing a short but perceptible hyaline portion, and male unknown. Aporcelaimellus hyalinus sp. n. is characterized by its body 1.49-2.23 mm long, lip region offset by constriction and 13-16 µm broad, odontostyle 14-16 µm long, neck 389-474 µm long, pharyngeal expansion 183-242 µm long or 45-53% of total neck length, uterus 44-106 µm long or 0.7-1.8 times the corresponding body diameter, V = 48-55, tail conical with rounded terminus (27-41 µm, c = 38-68, c' = 1.0-1.4) and short inner core, spicules 56 µm long, and seven irregularly spaced ventromedian supplements. Aporcelaimellus rarus sp. n. is distinguished by its body 1.96-2.34 mm long, lip region offset by constriction and 14-16 µm broad, odontostyle 15-16 µm long, neck 411-518 µm long, pharyngeal expansion 196-270 µm long or 48-52% of total neck length, a dorsal cellular mass at a short distance behind the cardia, uterus 63-96 µm long or 0.8-1.5 times the corresponding body diameter, V= 51-56, female tail short and conical (28-40 µm, c = 50-74, c' = 0.7-0.9) with rounded terminus and bearing a dorsal concavity at its posterior half, and males unknown. New data are provided for A. salicinus. A discussion of these atypical species is also provided.