Cryptophallus sinensis Liu sp. nov.Figs 1, 2Etymology.The specific name is an adjective derived from the Latin sinensis, meaning “ Chinese ”, referring to the species being found in Chinese waters.Material examined.Holotype: China • hermaphrodite; Guangdong Province, Huidong; 22°44.95'N, 114°45.05'E; 15 Apr. 2025; Hai-Long Liu leg.; sagittal sections on 8 slides; GenBank accession: PZ 111956 (COI), PZ 111958 (16 S), PZ 112129 (18 S) and PZ 112134 (28 S); MBM 288501. Paratypes: China • hermaphrodite; same data as for holotype; sagittal sections on 7 slides; GenBank accession: PZ 111959 (16 S), PZ 112130 (18 S) and PZ 112135 (28 S); MBM 288502.Description.Body is elongate oval, both ends are round (Fig. 1 A, B). Anesthetized specimens are 31– 25 mm in length and 15 mm in width. Dorsal surface is grayish brown, slightly darker in the median (Fig. 1 A), and the ventral paler. The extreme marginal of body is colorless. Brain and genital regions are reddish. A pair of small knob-like nuchal tentacles are located 6 mm from the anterior end and 1.7 mm apart from each other. Each has 5–8 tentacular eyespots. Numerous cerebral eyespots form a crowded cluster, within which two groups can be faintly distinguished (Fig. 1 C). Frontal eyespots are scattered over the anterior end of the body (Fig. 1 C). Marginal eyespots are distributed around the entire body.A ruffled pharynx is located at the center of the body, measuring 18–22 mm in length (Fig. 1 B). The mouth is located in the posterior region of the pharynx, approximately 5 mm from the posterior end of the pharynx (Fig. 1 B). The male pore is 6 mm from the posterior end of the body, and the female pore opens 2.5 mm behind the male pore (Fig. 1 B).The male copulatory apparatus is not enclosed by a muscular bulb and is comprised of the spermiducal bulbs, a free prostatic vesicle, and an unarmed penis papilla. A pair of vasa deferntia run ventrally along the outer sides of the uteri, and extend anteriorly to the region posterior to the brain (Fig. 1 B). With the musculature slightly increasing in thickness in the distal part, they develop into elongate tubular spermiducal bulbs that are difficult to distinguish from the vasa deferntia as separate organs. They converge near the proximal end of the prostatic vesicle to form an ejaculatory duct. The prostatic vesicle is very small and club-shaped, located beneath the posterior end of the pharynx, and positioned nearly vertically to the base of the penis (Fig. 2 A, E). The prostatic vesicle is lined with a simple ciliated epithelium and bears small epithelial folds at its proximal end (Fig. 2 C). The prostatic duct joins the ejaculatory duct near the tip of the penis papilla (Fig. 2 A, E). A wide, conical penis projects vertically into the male atrium, which is lined with ciliated epithelium (Fig. 2 A, E).Ovaries are situated dorsally. A pair of coiled uteri commence from the level of anterior end of the pharynx and run posteriorly along both sides of the pharynx (Fig. 1 B). The female copulatory apparatus is generally similar to that of other species in the genus, comprising a ductus vaginalis that lacks an outer opening of its own and opens together with the vagina into a common antrum (Fig. 2 B, E). The vagina extends upward from the female genital pore for a moderate distance, then runs anteriorly parallel to the ventral side of the body, curves sharply upwards and backwards, and after a short distance unite in a common oviduct (Fig. 2 B, D). Beyond this point, the ductus vaginalis loops back towards the female antrum (Fig. 2 B, E). In the sharply upward-curving portion of the vagina, it is twisted into a distinct spiral coil, making approximately five complete turns (Fig. 2 D). Cement glands are stained with acid fuchsin, empty into the portions of the vagina that run parallel to the ventral side of the body and that curve sharply upward (Fig. 2 B, D, E).Distribution.Huidong, Guangdong, China; Singapore.Habitat.Intertidal, under the stone.Molecular phylogeny.Cryptophallus sinensis sp. nov. is sister to a clade composed of Leptostylochus cf. gracilis and Leptostylochus victoriensis with poor support (51 / 0.54) and nested within Stylochoidea (Fig. 5; Suppl. material 1: fig. S 1).Remarks.As aforementioned, the revised genus currently comprises five species. Cryptophallus sinensis sp. nov. can be distinguished from its four congeners (except C. japonicus) by having a grayish-brown dorsal pattern with darker in the median region (Table 1). Additionally, the new species has fewer tentacular eyespots (5–8) than those in C. eximius, C. sondaicus, and C. wahlbergi (all> 13, Table 1). In C. aegypticus and C. sondaicus, the ejaculatory duct and prostatic duct open separately at the distal end of the penis, whereas in C. sinensis sp. nov. the ejaculatory duct connects distally to the prostatic duct (Table 1). Furthermore, C. eximius has the mouth opening in the anterior part of the body; C. wahlbergi possesses a prostatic vesicle with a proximally enlarged lumen (Bock 1913: textfig. 13), and C. sondaicus presents a rudimentary duct connected with the ductus vaginalis, which further distinguish these congeners from C. sinensis sp. nov. (Table 1). Cryptophallus sinensis sp. nov. differs from C. japonicus by its body color (grayish-brown dorsal pattern vs. light silvery black) and the arrangement of the cerebral eyespots (two scarcely separate clusters vs. a single cluster) (Table 1). In addition, the middle portion of the vagina forms a distinct spiral coil with approximately five complete turns in the new species, while in C. japonicus it is raised to nine (Kato 1938: fig. 6).The 28 S rDNA sequence of C. sinensis sp. nov. shows high similarity with that of Acotylea 9 (ZRC.PLA.0269, ZRC.PLA.0273) collected from Singapore (Ong and Tong 2018; Diong et al. 2025), with BLAST identities of 99.89 % (PQ 863117) and 99.79 % (PQ 863131), respectively. By contrast, the sequence identity between the two Singaporean Acotylea 9 specimens was 99.39 %. This level of intraspecific sequence variation, together with the matching color pattern (see fig. 16 A, B in Ong and Tong 2018) suggest that these specimens may be conspecific. Nevertheless, we acknowledge that this hypothesis should be further tested using additional molecular markers and detailed comparisons of internal morphology, due to the limited ability of 28 S rDNA to distinguish species (Oya and Kajihara 2017).