48 The nervous system of nemertean larvae has been studied so far only in pilidia, planktonotrophic larvae of nemerteans of the superorder Pilidiophora [1–3]. In contrast to larvae of other Trochozoa, pilidia do not have apical ganglia, which is regarded by some authors as the initial state of nemerteans . However, it was reported in the literature that, to make final con clusions, it is necessary to study the nervous system of larvae of nemertean of other groups, because pilidia are specialized larvae with catastrophic metamorpho sis . Hoplonemerteans (class Enopla or order Hop lonemertea in phylum Nemertea) have an atrochous lecithotrophic larvae developing either in water or in egg . Recent studies revealed a hidden metamor phosis in hoplonemertean larvae, during which the provisional epidermis is replaced with the definitive one [7, 8]. However, any data on the nervous system of hoplonemertean larvae are missing. This article con tains the first description of the serotonergic nervous system of hoplonemertean larvae exemplified by Qua sitetrastemma stimpsoni (Chernyshev, 1992). Mature males and females Q. stimpsoni were col lected in the intertidal zone of Peter the Great Bay (Sea of Japan) in August and September 2008. The ontogeny of this species is characteristic of the major ity of hoplonemerteans . Free swimming larvae hatched from eggs 12–14 h after fertilization. They had a round–oval body 220–230 μm in length with a long frontal tuft. One day after the hatching, larvae entered metamorphosis lasting for approximately one day, after which the juvenile hoplonemerteans 300– 400 μm long continue to float for another several days. We studies the nervous system of larvae at stages 12, 16, 20, 25, and 50 h after fertilization as well as juvenile hoplonemerteans 72 h old. The material was fixed in 4% paraformaldehyde prepared in phosphate buffer (PB) and stained with antibodies according to the fol lowing scheme. The fixed material was washed three times with 0.1 M PB (pH 7.5), placed into 50 μl of a solution of primary antibodies against serotonin (1 : 2000; Sigma, United States), and incubated at 4°C for 48 h. Then, the material was washed three times with 0.1 M PB supplemented with 0.025% Tween 20 and placed into a solution of rabbit second ary antibodies to mouse immunoglobulins labeled with fluorescein isothiocyanate (FITC, 1 : 600; Invit rogen, United States). The material was incubated at 4°C for 24 h, washed from excess antibodies with 0.1 M PB (pH 7.5) supplemented with 0.025% Tween 20, and embedded in 0.03% N propyl gallate (Sigma, United States) in glycerol. The wholemounts were analyzed under a Zeiss LSM 510 Meta confocal microscope. Serial optical sections cut with 0.4– 1.5 μm increments were recorded using the LSM 510 software, after which 3D reconstructions were gener ated. Since larva tissues, including the tegument, con tain a large number of vitelline granules that determine nonspecific fluorescence, wholemounts treated with nonimmune bovine serum instead of the primary anti bodies were used as a control.