Naidlds live in a wide range of aquatic habitats but are particularly important numerically as part of the benthic fauna of rivers with stony substrates. In general they graze on bacteria and algae although some, particularly Oiaetogaster spp., are mainly predaceous, and C. Hmnaei vaghini is a parasite of molluscs, chiefly Gastropoda. Food selection seems to be based largely on particle size although the food quality of the particles within the appropriate size-range influences rates of growth and reproduction.Major factors determining the distribution and abundance of naidid species are the nature of the substratum and the presence and kind of vegetation. Plants with a highly dissected form, a thick growth habit, and which permit the greatest pcriphyton development generally support the most abundant naidid populations. The oligochaete fauna of coarse substrates (stones and gravels) is often dominated by the Naididae but both species-richness and abundance of naidid populations are generally reduced where fine substrates (silts and muds) occur. The occurrence of worms within the substratum is also determined by its nature; naidids penetrate to depths of 20 70 cm in coarse substrates but rarely penetrate below 6 cm in mud. The principal factor limiting both depth penetration and the dominance of naidids in fine substrates is probably oxygen availability.Naidids reproduce both asexually and sexually, the former method predominating k>r most of the year. Asexual reproduction usually involves the budding-off of zooids but a few species fragment. Sexual reproduction is often infrequent; populations of some species produce few or no sexually mature mdividuals. In mature worms asexual reproduction virtually ceases. In populations that produce mature individuals there is apparently one sexual generation a year, usually occurring during the summer and autumn. Adults die soon after laying their cocoons.In general, naidids are most abundant during the summer months when rates of growth and asexual reproduction are stimulated by hi^er temperatures and a plentiful supply of food. A few species, however, e.g. Nais elinguis and Paranais litoralis, are more abundant in the spring.The response of naidid species to different kinds of pollution is varied but generally organic enrichment of rivers which have stony substrates results in a considerable (ten-to twenty-fold) increase in naidid abundance. Mi/s e//«p«s, N. barbata, N. communis, N. variabilis. and Chaetogaster diaphanus are often •Present address: Thames Water Authority, Reading HridKe House. Reading, Berkshire. Correspondence: M. A. Learner,

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1. The cocoon of Nais variabilis is ovoid with an average lengthof 527.6±8.4Mm (95% CL) and an average breadth of 378.1±9.9 /xm. The cocoon wall is about 7 /Ltm thick and the average volume of the cocoon cavity is 0.035±0.002 mm"*. Most cocoons contain one ovum; only single-ovum cocoons developed.2. Cocoons hatch in about 80 days at room temperature (average 20°C, range 17-23''C), Hatchlings emerged posterior end first and were, on hatching, about one-third the adult live length of 8.510.3 mm.3. It is estimated that adult Nais variabilis are unlikely to produce more than an average of ten cocoons each. Adults ceased asexual budding on becoming mature and died shortly after the completion of cocoon production. There was a brief co-existence of generations.