[Introduction] Lugworms are found, often in enormous numbers, in suitable beaches of muddy sand throughout the world. Their classification and geographical distribution have recently been revised in detail (Healy & Wells 1959, Wells 1957, 1959, 1961a, 1962, 1963). The purpose of the following essay is to summarise and discuss some of the main results of those papers. Lugworms are thick-bodied worms with rather thinner tails. They are currently referred to two genera, Arenicola and Abarenicola together with two genera of worms without specialised tails, they constitute the polychaete family Arenicolidae.