EARTHWORM, any terrestrial annelid of the order Oligochteta. Although certain fami lies of Oligochata are strictly aquatic and others exclusively terrestrial, still others con tain both aquatic and terrestrial species, and aside from peculiarities which are obviously adaptations to the mode of life no sharp dis tinctions can be drawn between these two classes. The earthworms generally are larger and more robust, with shorter sett than the waterworms and have dorsal pores. Within recent years the earthworms have attracted much attention from systematic writers and no less than 800 species, of which upward of 90 inhabit North America, are now known. Ex ternally they look much alike, but in their in ternal anatomy exhibit an astonishing variety. The common earthworms of Europe and America, of which a dozen or more species may be found in almost any suitable locality belong chiefly to the genera Lumbricus and Allolobophora which have the seta arranged in four pairs on each somite, a well-developed girdle or clitellum occupying a variable num ber of segments toward the anterior end, the female and male genital pores on the 14th to 15th segments respectively and various com plex internal peculiarities of the reproductive organs. While our species rarely equal a foot in length, several South African and Austra lian species reach five feet. Earthworms in habit nearly all parts of the earth except the frozen regions of high latitudes and altitudes, dry sandy soils and some portions of the North American prairies. Their habits are everywhere much alike. They burrow in damp earth, which the common kinds penetrate to the depth of about two feet. They swallow the soil and after digesting its nourishing elements reject the rest in the form of castings from the mouths of their burrows. At night they partly leave their burrows and draw to them the petioles of leaves, etc., on which they feed and with which they close the openings in cold weather. In this way they also seek one an other's company and copulate, as hermaphrod ites mutually fertilizing each other. A cocoon is formed by secretions of the clitellum and re ceives the eggs and spermatozoa in a quantity of albumen as it slips past the openings of the genital ducts toward the head, from which it passes and remains in the earth. During the
winter they burrow to a depth beyond the reach of frost, but some species at least will survive actual freezing. The importance of earthworms as cultivators of the soil can scarcely be overestimated. By their burrowing they render it porous and permeable to the rain and air; they continually turn the earth by bringing up soil from beneath the surface and they add to its fertility by burying vege table matter and by their secretions. Darwin has estimated that earthworms bring to the sur face in rich meadow lands not less than one fifth inch of soil per annum, and recent labor atory experiments demonstrate most emphat ically the beneficial influence on plants of the presence of earthworms.
The body wall of earthworms consists of an epidermis, one layer thick except on the clitellum; a layer of articular muscles; and an inner layer of longitudinal muscles. The cha ts: are shaped like a large S. The nervous system consists of a pair of dorsal cerebral ganglia, connected with a ventral ganglionated cord. Special sense-organs are absent, except for tactile cells, but touch, sensitivity to light and color-sense exists. There is a definite body-cavity, split up into separate compart ments for each segment. It communicates with the outside through dorsal pores. The vascular system is closed and consists primarily of a dorsal contractile vessel and ventral non contractile one, communicating by transverse contractile trunks in front and capillaries be hind. The excretory system consists of vari ously modified nephridia opening into the cce lum and to the outside. The digestive tube is usually straight and consists of a ventral mouth, a buccal cavity, a muscular pharynx, an oesophagus and an intestine opening by a terminal anus. Various glandular appendages exist and the whole system is often compli cated in several respects. The reproductive glands lie in the wall of the ccelum; their com plicated ducts are probably modified nephridia.
Consult Beddard, 'Earthworms and their Allies) (New York 1912) ; Darwin, 'Forma tion of Vegetable Mould) (London 1881).