Page:The New International Encyclopædia 1st ed. v. 06.djvu/681

* EARTHWORM. 593 EARTHWORM. inj; the bluntly pointed lieaii between the par- ticles of earth and pre^i^iinJr them apart, but also by actually takinj; into the inoutli and swallow- inji imieh of the opposing material. In all their movements, the short but still' .«tles alons; the sides of the body are of the <;reatest assistance. These seta^ are arranjjed in pairs on each segment, two pairs on each side, so that evcr_' segment carries eiglil seta', and they form two double, longitudinal rows along each side of the worm. Earthworms form the prey of many animals, from slugs upward, and form the |irin- cipal food of moles, shrews, and the like. Their habit of going into the water, and the accidents of floods, cause many to fall a prey to lishcs; and they are the conunonest bait for stilltisliing, whence they are more commonly known in the Inited .States as 'angle -worms' than l)y any other name. E.KTHvoKMS Axn SoiL. The important part that earthworms jday in the economy of nature was not generally recognized until attention was called to it by Charles Darwin. Jlost of our knowledge of the habits of these creatures is due to him. Eartliworms are vegetable feeders ex- cdusively. In burrowing the worms take more or less earth into the alimentary canal, where it is mixed with the vegetable food of the worm, and'nearly all of it passes out with the cast- ings, which are always voided on the surface. These castings are therefore a sort of natural 'compost.' which the earthworms are continually distributing over the surface of the areas which they inhabit. They are thus not only cimstantly transforming vegetable matter into more usal)le forms, but are also so mixing it with inorganic matter as to make both of greater use. Such a process is called the formation of vegetable mold, and it is continually going on wherever the conditions are suitable. It is not, of course, confined to the agency of earthworms, though Mr. Darwin showed that they were one of the most imi)ortant agents. The rate at which veg- etable mold may be formed by these worms ^vill vary of course with the character of the soil, the number of worms, and the abundance of food, but under favorable conditions it may be sev- eral inches in a century. Such soil is fertile not alone because of its composition, but because the movements of the worms keep it stirred up and well aerated. For further particulars in reference to the habits and usefulness of earth- worms, consult Darwin. The Formation of Vege- table Mould Tlirouh the Action of Worms (I.im- don, 18S1). Strlctire. Earthworms are generally of a uniform color, although there is a ditl'ercnce in shading between the upper and under surfaces and between different regions of the body. The color is usually tiesh-red. varying into dull pink on the one hand and dirty brown on the other. In size there is great diversity, some species reaching a length of only a few inches, while tropical species may be several feet in length. Large specimens of the common .merican species are rarely a foot long. In such s])ecinicns as many as 180 segments may be present, but 130 is about the average number. The muscular sys- tem of earthworms is well developed, and con- sists of an outer series of circular or transverse muscle-fibres which girdle the body, and an inner series of longitmlinal fibres which form five principal bands, and several suuiller ones concerned with the movements of the seta". The nervous system consists of a large ganglion above the tt'sophagus. often called the brain, and a ventral cord, which lies beneath the alimentary canal, and bears ganglia in every segment. This cord is connected with the brain bv conunis- STRUCTUBE OF AN EARTHUOK.Nf. 1. .ii entire worm, side vipw : 1, Isl .segment and nioiltb; l."). ;i.'Stli seg-nient, showing opening- of male g:enital ducts; a:i. 3:id segment and tliicliened'clitellum.' 2. .Anterior por- tion of Lunibriclls terrestris. laid open and eidargpd ; o, mouth :/>, pharynx; /<. gullet; /«. (esophageal glands ; r, proventricuius: s, gizzard; i, intestine; li, one of the • hearts" t)orne on the side of the dorsal vessel ; t. seminal reservoirs; sp, spermathecte. (After Lankester.) sures around the oesophagus. So far as known, there are no sense-organs other than those of touch. The alimentary canal consists of a muscular pharynx, a slender a?sophagus. a mus- cular gizzard and thiu-wallcd crop, and a long, straight intestine, within which is a complex dor- sal fold, to increase the digestive surface. The circulatory system is well developed and con- sists of a prominent dorsal blood-vessel and no less than four ventral vessels, which run Ion- gittidinally in the body, and are connected with each other by a regularly arranged series of transverse vessels, many of which form a net- work of capillaries in the nuiscles and in the wall of the intestine. In some of the anterior segments some of these transverse vessels are much larger than elsewhere, and form very ])rom- inent arches, often called "hearts,' of which there are usually five pairs. They are not functionally hearts.' however, or at least only in |)arl. for most of the pinn)ing is done by the dorsal trunk and general muscular movements. Excretion takes place through the very characteristic nc])liridia. of which there arc a pair in each segment, oiu' on each side of the intestine. They consist of coiled tubes, having at one end a funnel-shaped ojM'ning into the body-cavity, while the other end opens outside of the body. The fuimel always opens into the segment just anterior to that in which the tube itself lies. Ui;pi;ouucTiN. Earthworms are hermaphro- ditic — that is. both nuile and female repro<luclive organs occur in the same individual. Hut nuitual cross-fertilization probably always takes place. The eggs are laid in capsules which are buried in the earth and serve to protect the young un til they are well developed. These capsules are probably formed from the secretion of the rlitrl- lum. a peculiar thickening of the body-wall, be- tween the twenty-ninth and thirty-fifth seg- ments. The clitellum is generally a very promi- nent band, forming one of the most noticeable