Many species of polynoids have been famous for the "conrnensal" habit among Polychaeta. The author can count 274 species of cornmensal (or may be commensal) Polychaeta in 34 different families in literature, and 40% of them in polynoids. They are classified into three ranks. Rank A is for the Polychaetes almost always foufhd in commensal condition with hosts. Rank B is for the species not only frequently found in commensal but also in free living condition. Rank C is for the species almost free living, bur sometimes in commensal. The polychactes of strong tendency toward commensal life (A+B=t) are 154 species, and the species of the family Poiynoidae are half of them (72 species).
Many groups of marine organisms are well known as involving of many commensal or parasitic species. But almost all the groups are classified as the differentiated or modified situation rather than the primitive one in each higher taxa. Polynoidae is said to one of the most primitive families in Polychaeta, and it is the largest family in Polychaeta. This is he most peculiar point of Polynoidae comparing with other commensal groups. But commensal condition of Polynoe dae is rather similar to that of the shrimps of the family Alpheidae.
The setal modifications for comtneusal life in polynoids may be three characters. 1) Basal cusp in neurosetae, 2) Hook-shaped neurosetae to anterior parapodia, 3) short, stout neurosetae with reduced teeth on their free margin. Many genera known to contain commensal species, are applicable to some of three characters mentioned above, but some genera dissatisfy any of those. In opposition, two genera, weberia Horst 1915 and Paraholoepidella Pettibone 1965 never rerorded commensalism, satisfy the third character, therefore, these two genera my be commensas. Adaptive color patterns are also conspicuous in many species. Ventral lamellae aid unusual development of ventral cirri are also characteristic in some commensal species.
Poiynoid comensals can be divided into two types, lodging type and clinging type. The former is for the species living in tubes or burrows of polychaetes, holothurians and other marine organisms. The latter is for the species clinging on the body surface of echinoderms, alcyonarians, etc. Polynoids of the subfamily Lepidonotinae prefer to lodging type, and that of the subfamily harmat;loinae to clinging type. Therefore, the range of host types are different between two subfamilies. The species in Lepidonotinae frequently prefer to sedentary polychaetes as theirr hosts, the other hand the species in Harmothoinae usually prefer to echinoderms and anthozoans.
It seers reasonable that morphological and color adaptive changes for commensal life are necessary for the clingings rather than the lodgings. Therefore, Hamothoinae has more adaptive modifications in setal morphology or ventral structures than Lepidonotinae.Furthermore, the former has more genera consisted of only commensal species than the latter.

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Previous studies (Nishihira et al., 1980; Sato et al., 1982; Tsuchiya et al., 1982) on the breeding ecology of the polychaete, Lumbrineris latreilli (Audouin et Milne-Edwards) were summarized and reproductive strategy was discussed. The worm was restricted to the patches of Zostera marina L. on the fine sediments and patches of Sargassum and other algae growing on pebbly bottom. When water temperature rises up to about 16°C, many breeding individuals come up on the algae to liberate gavietes around high tides at night. The worms select spawning site on the sea weeds growing at, or just near, their living place, and tend to attach egg masses to the sea weeds as high as possible. The local population in a certain area spawns almost simultaneously (2-3 days). Fertilized eggs develop into juveniles through trochophore within a jelly mass and the young emerge little by little over a period of about 20 days in a closed shore, so the stage of development at emergence varies from 3- to 10-setiger stage. It was estimated that 60-70% or more of spawn eggs became potential recruits around the spawning site, but the number of young actually collected in the field was about 1/3 of the estimated figure.

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Population dynamics on Capitella capitata has been studied in Amakusa since November 1979. Though the population reproduced almost throughout the year, the seasonal fluctuations of population size were very large. Size cohorts were detected in the size distribution, because spawning occurred synchronously. Their life cycles were traced by the close quantitative investigations. The prereproductive period was for 4 to $ weeks and b cohorts occurred only for 4 months. The survivorship curve was estimated by the new technique, using computer simulation.

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Eight grain-size fractions of the sediments were separated in Hakodate Harbor. Both total carbon and total sulfide contents in the sediments were proved to be not relating to particle diversity. Differences in annelid species diversity were attributable to variations in both, species richness and evenness. Species diversity was not correlated with total carbon and total sulfide, but correlated with particle diversity. If particle diversity is explained as showing increased habitat heterogeneity and ingestible food variety, the latter two may regulate the organization of several species assemblages of various feeding- and mobility modes.

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Analytical study was carried out on the factors which regulate the ecological distribution of the mobile polychaete Armandia sp. on an intertidal sand flat. The larval settlement on the particular zones of the flat. is determined by the three processes: passive accumulation of the larvae by slowdown of the transporting force of the water current or the wave, active selection of the substratum by the settling larvae, and predation by the hermit crab pioaenes nitidimanus and exclusion by the ghost shrimp Callianassa japonica. In the summer season when the sea water on the flat is calm, the subsequent population movement after settlement on the flat can be regarded as a random dispersion process on a one-dimensional space with an incomplete reflection boundary due to sand ejecting activity of Callianassa. In winter when the northery wind prevails, the vigorous incoming waves together with the flow tide expel the animals from the surface of the sand and carry them to the landward end of the flat.

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