(Figs. 7–9)
Material exmained. Holotype (PLA-Ma0021): permanent slides of longitudinal section. Paratype (PLA-Ma0022- 23): permanent slides of longitudinal and horizontal sections. All specimens are deposited in IZCAS. The samples were collected at the same site as that of M. heyuanensis n. sp.
Etymology. The name of the new species is originated from the name of Dongyuan County, Heyuan City, Guangdong Province, China.
Description. The body is flat, transparent and in milk white color. The length and width of its body is 2639 ± 274.2 µm (n = 4) and 631 ± 108.4 Μm (n = 4), respectively (Figs. 7 A–D, 8 A–B, 9 A).
M. dongyuanensis n. sp. is hermaphroditic. The male reproductive system has a pair of elliptical-shaped testes (t), which is located at both sides of intestine. The morphology of the sperm (Figs. 7 J, 9 E) is similar to that of M. lignano (Schärer et al. 2011). When adult M. dongyuanensis is swimming, the false vesicula seminalis (fvs), vesicula seminalis (vs) and vesicula granulorum (vg) are arranged from the anterior- to posterior-part along the mediventral line (n> 10, Fig. 7 F). The posterior side of vesicula seminalis (vs) is connected with a “U”-shaped vesicula granulorum (vg). The posterior end of vesicula granulorum (vg) is connected with and partially penetrated into penis stylet (ps) tube (Figs. 7 G, 8 E–F, 9 A–C). Penis stylet (ps) is bended in a “J” shape planimetrically. The distal opening points at the male gonopore (mg). The curved-line (marked as ‘a’) and straight-line (marked as ‘b’) distances between the base and distal end of penis stylet (ps) are 157 ± 20.78 Μm (n = 5) and 152 ± 23.58 Μm (n = 5), respectively (Fig. 4 D). The diameters of basal and distal parts of penis stylet (ps) are 27 ± 4.15 Μm (n = 5) and 4.4 ± 0.50 Μm (n = 5), respectively. (Fig. 9 D). The distal end of penis stylet (ps) has no spine nor thickening. The features of the female reproductive system are identical to those of other species within the genus of Macrostomum.
Remarks. A dozen species are similar to M. dongyuanensis n. sp., i.e., M. bulbostylum Kepner & Stiff, 1932, M. curvituba Luther, 1947, M. axi Papi, 1959, M. balticum Luther, 1947, M. amurense Beklemischev, 1950, M. quiritium Beklemischev, 1951, M. georgeense Young, 1976, M. parmum Ball, 1977, M. magnacurvituba Ax, 1994, M. sinensis Wang, 2005, M. spec. 1 Ax, 2008 and M. spec. 2 Ax, 2008. The penis stylets (ps) of these species are all “J” shaped. Among them, the stylet distal ends of 5 species of Macrostomum, i.e., M. axi, M. balticum, M. sinensis, M. spec. 1 and M. spec. 2, are spine-shaped. In contrast, the distal end of the penis stylet (ps) in M. dongyuanensis n. sp. has no affiliated spine structure. As for M. magnacurvituba, M. bulbostylum, M. curvituba and M. quiritium, the distal ends of their stylets are clearly thickened.
In M. georgeense, the base of penis stylet (ps) is slightly bent, the distal end is bent in a 90 ° angle, and the width of its anterior half is identical. In M. parmum, the length of penis stylet (ps) is 160 Μm. It prolongs in a funnel-shape and situates parallel to the mid-ventral line of the body, while the distal end is thickened and bends rightwards in a 90 ° angle. As for M. amurense, its penis stylet is similar to M. georgeense, except that its distal opening bends leftwards and has a groove. In contrast, the basal end of the penis stylet (ps) in M. dongyuanensis n. sp. is slightly bent. The distal end is bent in an obtuse angle, and has no spine nor thickening structures.
To sum up, the morphology of the penis stylet of M. dongyuanensis n. sp. is distinct from those of the above 12 similar species.