ABSTRACT
The 17,000 speciose annelids are known for the unique, spectacular epitoky, regeneration and clonal reproduction. They are found in marine (81%), freshwater (12%) and terrestrial (9%) habitats; some of them are subterranean stigobionts and some others inhabit hydrothermal vents. The distribution extends from 4,900 m depth in the oceans to 2,000 m altitude in the montane lakes. Description of life history of polychaetes is limited to 3% of them and is increasing only at the rate of 3 species/ year. For the first time, relevant information is highlighted on gutless oligochaetes and polychaetes, anaerobiosis, osmotrophism and acquisition of nutrients from symbiotic microbes. For example, constantly migrating between the nutrient-rich anoxic and oxyic zones in sediments, the gutless tubificds have colonized an ecological niche, so far unoccupied by any other interstitial fauna. In the absence of exoskeleton, ~ 50% of polychaetes are brooders and all the clitellates (oligochaetes + hirudineans) oviposit cocoons.
