HABITAT.—Earthworms are found in almost every part of the world where they have been looked for. They occur far to the north, in Siberia and Nova Zembla,[419] while South Georgia and Kerguelen mark their southern limits. From arid tracts of country they are naturally absent, and also, which is more {366}curious, from certain districts of North America. In the tropics these animals seem to be on the whole less abundant than in more temperate climates. But this deficiency of individuals is counterbalanced by the greater variety of generic and specific types. From tropical Africa, little explored as it has been from this point of view, no less than thirty genera, including about ninety species, have been recorded; whereas in Great Britain only four genera and seventeen species occur, and in all probability but few remain to be discovered. The vertical range of these Annelids is also considerable. Several species have been met with in Europe and elsewhere at an altitude of 10,000 feet. For the bulk of the species the term earthworm is an accurate description of their habitat. But there are not a few which occasionally or habitually prefer other localities. The genus _Allurus_ is equally at home in soil or in water; I have taken it in the fast-flowing river Plym in Devonshire. The genus _Acanthodrilus_ includes a few species which have at present only been met with in water; _A. schmardae_ comes from fresh water in Queensland, _A. stagnalis_ from ponds in South America; _A. dalei_ is like _Allurus_ in that it is to be found both on land and in streams and ponds. The Enchytraeidae are just as amphibious; _Criodrilus_ and _Sparganophilus_ appear to be purely aquatic. A more curious locality for a creature that is so characteristically terrestrial is the margin of the sea. For a long time a species belonging to a peculiar genus _Pontodrilus_ has been known from the shores of the Mediterranean in the neighbourhood of Nice. It lives there among seaweed above high-water mark, but it must at least occasionally be splashed by the waves. Another species of the same genus occurs on the coast of Brazil and some of the West Indian islands; _Pontoscolex corethrurus_ and _Diachaeta littoralis_ were described by Schmarda[420] from the shores of Jamaica. The former species is one of the most widely distributed of earthworms, and, except in this particular part of the world, has been always taken on the land far from the sea. There are also partly marine forms among the Tubificidae; _Clitellio arenarius_ is common on our coasts.