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.