It was considered advisable, whilst describing the Polychaeta from the Chilka Lake, to examine also those species of the group in the Indian Museum which, at various localities in India, had been found in water which was either fresh or periodically mixed with fresh water, or in other words was not distinctively marine. The collection contains very few species which spend their whole lives in fresh water. The majority live either in brackish water of low salinity, or belong to the "Euryhaline " group. The latter term was first used by Moebius to designate those species which can live in water the salinity of which varies between wide limits. In Europe very few species of Polychaeta can tolerate marked changes in the salinity of the sea-water and fewer still can reproduce under such conditions, but in India, judging from the list of species dealt with in the present paper which is obviously far from exhaustive, they are relatively far more numerous. This adaptation may be correlated with the sharp division of the climate into wet and dry seasons, whereby the littoral region is periodically flooded with water of low salinity, especially in bays and estuaries.
From the whole of India only two species of Polychaeta have been recorded from fresh or brackish water. These are Matla bengalensis Stephenson (1908, p. 39 and 1910, p. 82) and Spio bengalensis Willey (1908, p. 389) both from brackish pools at Port Canning, Lower Bengal. Matla bengalensis is based on juvenile specimens of a Capitellid, and neither it nor Spio bengalensis were represented in the present collection. Records of littoral marine Polychaeta from Indian shores are practically absent. A number of species have been recorded from Ceylon by Schmarda, Grube, Michaelsen, and Willey. It is therefore not surprising that almost all the species in the present paper are new to science, especially when one remembers the peculiar nature of the habitat in which they have been found. Many of the species of Polychaeta known from the Indian Ocean and South Pacific have been very imperfectly described and inadequately figured, judged by modern standards. Any specimens now referred to such species would be involved in a cloud of uncertainty, and it seems preferable, where there is any doubt, to ignore them until they have been redescribed from trustworthy material.