First study of the Polycladida (Rhabditophora, Platyhelminthes) from the Pacific Coast of Costa Rica Author Soutullo, Patricia 0000-0002-9691-0038 Dept. Biodiversidad y Biología Evolutiva, Museo Nacional de Ciencias Naturales (CSIC). José Gutiérrez Abascal 2, Madrid, Spain. Patricia Soutullo: patriciasoutullo @ gmail. com; https: // orcid. org / 0000 - 0002 - 9691 - 0038 patriciasoutullo@gmail.com Author Cuadrado, Daniel 0000-0002-9691-0038 Dept. Biodiversidad y Biología Evolutiva, Museo Nacional de Ciencias Naturales (CSIC). José Gutiérrez Abascal 2, Madrid, Spain. Patricia Soutullo: patriciasoutullo @ gmail. com; https: // orcid. org / 0000 - 0002 - 9691 - 0038 patriciasoutullo@gmail.com Author Noreña, Carolina 0000-0002-9691-0038 Dept. Biodiversidad y Biología Evolutiva, Museo Nacional de Ciencias Naturales (CSIC). José Gutiérrez Abascal 2, Madrid, Spain. Patricia Soutullo: patriciasoutullo @ gmail. com; https: // orcid. org / 0000 - 0002 - 9691 - 0038 patriciasoutullo@gmail.com text Zootaxa 2021 2021-04-22 4964 2 363 381 journal article 7069 10.11646/zootaxa.4964.2.7 c36b481b-c2c3-4f7c-b5b9-614607673bb8 1175-5326 4709585 7E356498-C414-4B75-A3E5-5E9E85291BBB Marcusia ernesti Hyman, 1953 ( Fig. 6 ) Material examined : One specimen. Sagittal histological sections stained with AZAN of one individual. 42 slides: MNCN 4.01/2461 to MNCN 4.01/2502. Location: Playa Carbón Beach , in Las Baulas National Marine Park of Guanacaste , Costa Rica , 10 ° 20’53.4 “N 85 ° 51’44.5” W ; May 87 2018. Fig. 6A . Habitat: Low intertidal, below stones on sand Description : External features ( Fig. 6B,C ): Oval-elongated body shape, wavy margins, anterior and posterior end rounded. Small marginal tentacles in the frontal region, separated. Length: 23 mm . Dark brown almost black dorsal colouration with white irregular spots. Whitish ventrally. Colourless region at cerebral eye level. Pharynx ruffled large located in the central area. Reproductive system: Consists of a common elongated atrium in which the male atrium opens laterally and the female distally ( Fig. 6D ). The male copulatory organ is formed by an elongated muscular bulb that encloses the seminal vesicle and the penis papilla. The ejaculatory duct is lined by a well-developed prostatic glandular epithelium ( Fig. 6C ). Without prostatic vesicle. The female apparatus includes the vagina that turns caudally and widens, covered with glandular epithelium. Distribution: Marcusia ernesti has been found only in the Pacific Ocean. This species is known from Baja California , Gulf of California, Sonora ( Mexico ), Clarion Island ( Mexico ) (Hyman, 1953) and now from Costa Rica in this study.