The paper outlines the current discussion on the species problem : what actually is a species and how do we recognize such an entity in nature? Six of the major species notions are briefly explained. A distinction can be made between species concepts based on the process of interbreeding and concepts that focus on common descent. Application of these different types of species concept may give incompatible results. It is discussed to what extent we can be certain that the species delimited represent objective realities in nature and are independent of the discovery procedure applied. Further, it is argued that because of the existence of various evolutionary processes, nature can be divided into several natural entities, each representing a species of some sort. The choice of a particular species concept is determined by that aspect or process in nature that forms the focus of one's research agenda. This plea for pluralism should preferably not affect taxonomic procedure ; named species taxa should be based on only one, taxic species concept.

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Fourteen new species of dicyemid mesozoans, included within four genera, are described from six shallow-water species of Japanese cephalopods : Dicyema colurum sp. nov. and Dicyema erythrum sp. nov. from Octopus fangsiao ; Dicyema dolichocephalum sp. nov. and Dicyema sphyrocephalum sp. nov. from Octopus minor, Dicyemennea gyrinodes sp. nov., Dicyemennea ophioides sp. nov., and Dicyemennea trochocephalum sp. nov. from Octopus hongkongensis ; Dicyema lycidoeceum sp. nov. and Pseudicyema nakaoi sp. nov. from Sepia lycidas ; Dicyema hadrum sp. nov., Dicyema rhadinum sp. nov., Dicyemennea mastigoides sp. nov., Dicyemennea minabense sp. nov., and Pseudicyema nakaoi sp. nov. from Sepia esculenta ; Dicyemodeca anthinocephalum sp. nov. from Octopus dofleini. The status of Pseudicyema and Dicyemodeca is briefly discussed and the validity of these two genera is reaffirmed. The majority of the dicyemid species examined was found to be host-specific. In a few instances the same species of dicyemid was detected in two different cephalopod host species belonging to the same genus. Host specificity appears to be characteristic of dicyemid-cephalopod relationships. The largest cephalopod host species, namely Octopus dofleini, Octopus hongkongensis, and Sepia esculenta, harbored dicyemid species with the longest or largest vermiform stages. Two new types of cells are described from the infusoriform embryos of Dicyemennea gyrinodes and Dicyemodeca anthinocephalum. Three additional cell numbers, namely, 41, 42, and 43, are recognized from the infusoriform embryos of Dicyemennea gyrinodes, Dicyemodeca anthinocephalum, and Dicyema dolichocephalum, respectively. The cell types and morphology of infusoriform embryos are briefly discussed.

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Four species of the genus Hypatima Hubner, H. pentagonia sp. nov., H. acicula, sp. nov., H. ovata sp. nov., and H. stenosa sp. nov., and a species of Capidentalia Park, C. khaoensis sp. nov., are described as new to science from Thailand. Six species belonging to the genus Hypatima and allied genera, H. anguinea (Meyrick), H. iophana (Meyrick), H. arignota (Meyrick), H. haligramma (Meyrick), Faristenia polemica (Meyrick), and Dendrophilia (Dendrophilia) hetaeropsis (Meyrick), are reported for the first time from Thailand. Descriptions and illustrations of male and female genitalia for the known and new species of the genera Hypatima and Capidentalia are provided, and brief taxonomic data for the species of Faristenia and Dendrophilia are given.

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A redescription of Canthocamptus iaponicus Brehm is given, and three new species from Japan are described : Canthocamptus kitaurensis, C. macrosetifer, and C. takkobuensis. Canthocamptus iaponicus and C. kitaurensis are close to the North American species C. staphylinoides and the Asian species C. carinatus. The other two new species are close to the Eurasian species C staphylinus and C. microstaphylinus. The distribution ranges of these species in Japan as well as a key are presented.

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A new phyllodocid species, Nereiphylla hera sp. nov., is described from Hokkaido, northern Japan, as the second Japanese species of this genus. The new species is distinguished from its congeners by flat, spatulate tentacular cirri with distinct tips.

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A new pycnogonid species, Achelia cuneatis, is described and illustrated from specimens collected in shallows of northeastern Sakhalin Island, Russian Far East. One known species, Nymphon brevitarse Krφyer, is also reported from a few specimens found in the same locality. The new species is compared with two congeners known from northwest Pacific waters.

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A new species, Hypebaeina milkoi sp. n., from Kirghizia is described. A diagnosis and figures of male genitalia and urites are given. A distribution map is presented.

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Nephropsis macphersoni is newly described based on two specimens, male and female, from the bathyal zone in Australian waters. One of the specimens was previously designated as the paratype of N. holthuisi Macpherson, 1993. To clarify the taxonomic status of N. holthuisi in the genus, it is redescribed on the basis of the holotype and additional specimens mainly from Japanese waters.

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Kishinouyea corbini Larson, 1980 is recorded from Santa Cruz, Espirito Santo State, southeastern Brazil. This is the first record of the order Stauromedusae from Brazil, and also from the tropical Southern Hemisphere. Kishinouyea corbini has been known only from two localities in Puerto Rico, and this new record constitutes a great southward extension of the known range of the species. This is also the first report of the species since its original description, so a description of the Brazilian specimens and a comparison with the type material are given. Records of Stauromedusae in the Southern Hemisphere are briefly reviewed.

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The North Pacific prickleback Alectridium aurantiacum (Teleostei : Stichaeidae) is redescribed on the basis of 21 specimens collected from the central Kuril Islands. Special attention is given to the distribution of the cephalic sensory canals and pits. These specimens represent the first known records of this species outside the Commander and Aleutian Islands.

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