We describe a new species of polyclad flatworm, Enchiridium daidaisp. nov., from the rocky subtidal zone in the East China Sea along the coasts of the Kyushu and Okinawa Islands, Japan. Enchiridium daidaisp. nov. is characterized by i) the entire periphery of the dorsal surface narrowly fringed with orange, ii) a marginal-eyespot band extending to the position of the mouth (about anterior one-eighth of body), and iii) two prostatic vesicles covered by a common muscle sheath, which is penetrated by the ejaculatory duct. We performed a molecular phylogenetic analysis based on 945-bp 28S rDNA sequences of 16 species of Prosthiostomidae currently available in public databases in addition to those of E. daidaisp. nov. and Prosthiostomum torquatumTsuyuki et al., 2019. In the resulting tree, our new species was nested in a clade composed of Enchiridium species. The tree topology was in favor of a taxonomic view that Enchiridium should be defined by having i) a common muscle sheath that encloses two prostatic vesicles and ii) marginal eyespots that may or may not surround the periphery of the dorsal surface.

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We describe a new species of polyclad flatworm, Pericelis flavomarginata sp. nov., from the intertidal and subtidal zones along localities on the Pacific coast of Japan. Pericelis flavomarginata sp. nov. is characterized by i) the dorsal surface of the body fringed by a lemon-yellow line except for the tip of tentacles, with a narrow brown midline running from the anterior edge of the body to the posterior end of the pharynx, ii) the pair of marginal tentacles with the tips extending and tapering, and iii) the presence of a common gonopore. Our molecular phylogenetic analysis revealed that selected Pericelis species were divided into two clades, each of which may be agreed with a characteristic dorsal color pattern. Additionally, we report an observation on the feeding behavior of P. flavomarginata sp. nov. on the polychaete Iphione muricata (Savigny in Lamarck, 1818).

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We describe a new species of ilyplanid polyclad, Zygantroides serpulidicola sp. nov., from Amakusa, Kumamoto, Japan. This is the third member of Zygantroides Faubel, 1983 and the first record of the genus from the Pacific Ocean. Zygantroides serpulidicola sp. nov. is characterized by i) the mouth opening near the common gonopore, ii) the sperm ducts separately entering a pear-shaped seminal vesicle, iii) an elongated Lang's-vesicle duct, and iv) the horseshoe-shaped Lang's vesicle located posterior to the common gonopore. We provide a partial sequence of the mitochondrial cytochrome c oxidase subunit I (COI) gene (712 bp) as a DNA barcode for the species. Our phylogenetic analyses based on concatenated sequences of the 16S, 18S, and 28S ribosomal DNA and COI indicate that Z. serpulidicola sp. nov. is nested in a clade comprised of Discocelidae and Ilyplanidae; the latter does not appear to be monophyletic.