Fig. 194.—Cocoons of Lumbricidae. (After Vejdovsky.) A, Lumbricus rubellus, nat. size and × 3; B, Allurus, nat. size and × 6; C, Allolobophora foetida, nat. size and × 3.
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 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.
While there are several kinds of earthworms that are thus met with in fresh water, others will live for some time submerged in water. Perrier found by experiment that various species could undergo with impunity a prolonged immersion in water, and I confirmed his experiments myself with a common species of Allolobophora. A correspondent of "Nature" stated that a certain number of species (not particularised) of earthworms in Ceylon could suffer with impunity the effects of sea-water. The importance of this fact will be again dealt with in considering the geographical distribution of the group.
Among the aquatic genera of Oligochaeta we do not as a rule meet with amphibious species. The Enchytraeidae however, as already mentioned, are an exception; so too appears to be the genus Phreoryctes, which in its structure is to some extent intermediate between the earthworms and the aquatic families.
Terrestrial and Aquatic Forms.—There are many obvious structural peculiarities which would prevent the normally aquatic worms from being thoroughly at home on dry land. The gills of Branchiura and the other gilled species would be injured, in all probability, by friction with the earth; the delicate and long chaetae of Naids and Tubifex are also most unsuited for progression through dry soil; and it is to be noted that those Oligochaeta, which, belonging to aquatic groups, are yet found away from water, have chaetae of the simple sigmoid pattern which characterises the earthworms.
There are other peculiarities found only in the aquatic species which have not so obvious a relation to their habitat. In no genus that is mainly aquatic in habit are the ova small and nearly unprovided with yolk as in Lumbricus; the ova of aquatic forms are invariably large and filled with abundant yolk.
The more delicate organisation of the aquatic Oligochaeta is not so hard to understand. The comparatively unresisting nature of the medium in which they live, water or fine mud, does not necessitate so strong a development of the layers of the body-wall as is essential to the earth-living forms, which have also thick septa in the anterior region, to protect the organs of reproduction as the strong muscular contractions of the body force the worm's way through the dense soil. With the weak structure of the integument are perhaps also correlated the simplicity of other organs of the body in the aquatic Oligochaeta. With thin body-walls, through which gases can diffuse with great ease, there would seem to be less need for the development of a system of integumental blood capillaries. These are indeed for the most part absent in the aquatic forms, being only faintly developed in a few, an example possibly of degeneration.
Earthworms and the Soil.—Darwin has explained the enormous effects which these soft-bodied and small creatures have had upon the superficial structure of the earth. Their castings, brought up to the surface, are blown about by the wind when dry, and are thus spread over the ground in a fine layer. It has been calculated that in the space of an acre .2 of an inch in thickness of earth is annually brought to the surface. It is clear therefore that in a long period of years there would be a very large effect produced. On the sides of a hill this matter brought up from below would tend to roll down the slopes when dry, and would increase the débris carried away to the sea by streams and rivers, so that continents formerly deposited under the sea may owe no small proportion of their size to the continued work of earthworms in past ages.