Observations on the occurrence of Asellus (Isopoda, Crustacea) in some lakes of northern Britain

Summary

1.	The distribution of Asellus in 65 standing waters in northern Britain shows three zones, unfavourable where it is usually absent, marginal where it may be present or absent and favourable where it is invariably present. These show a relationship, probably of an indirect kind, to the calcium and dissolved matter contents of the water. Dispersal by human activities seems unimportant.
2.	These conclusions are in marked contrast to those of MOON (1957) for the Lake District area. His data however can be fitted into the above scheme.