STRUCTURE. Earthworms are generally of a. uniform color. although there is a difference in shading between the upper and under surfaces and between different regions of the holly. The color is usually Ilesh-red. varying into doll 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 he several feet in length. Large specimens of the common American species are rarely a foot long. In such specimens as many us segments may be present, but 130 is about the average number. The museular sys tem of earthworms is well developed, and con sists of an outer series of eireular or transverse musele-fibres which girdle the body, and an inner series of longitudinal fibres which form live principal bands, and several .111a11(.1. ones concerned with the movements of the seta-. The nervous system consists of a large ganglion above the (esophagus. often called the brain. and a ventral cord, whielt lies beneath the alimentary canal. and hears ganglia in every segment. This cord is connected with the brain by commis sures around the (esophagus. So far as known, there are no sense-organs other than those of touch. The alimentary canal consists of a muscular pharynx. a slender (esophagus, a Inns cular gizzard and thin-walled 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 lon gitudinally 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 muscles and in the wall of the intestine. In some of the anterior
segments some of these transverse vessels are much larger titan elsewhere. and form very prom inent arches. often called 'In-arts,' of which there are usually live pairs. They are not functionally 'hearts.' however, or at least only in part, for most of the pumping is done by the dorsal trunk and general muscular movements. Excretion takes place through the very characteristic nephridia. of there are a pair in each segment. one on each side of the intestine. They consist of coiled tubes. having at one end a funnel-shaped opening into the hody-cavity, while the other end opens outside of the body. The funnel always opens into the segment just anterior to that in which the tube itself lies.
1;Ereoot1 vox. Ea rthWOrilis are hermaphro ditic—that is, both male and female reprodnetive organs occur in the same individual. But mutual cross fertilization probably always takes place. The eggs are laid in capsules which are buried in the earth and serve to protect the young 1111 ill they are well developed. These capsules are probably formed from the secretion of the elitcl 1nm, a peculiar thiekening of the body-wall, be tween t he twenty-ninth and thirty-fifth seg ments. The elite] 111111 is generally a very promi nent hand. forming one of the most noticeable of the external features of an earthworm, but it is not always evident. The eggs of earthworms contain considerable yolk. and the young worms develop without any metamorphosis.
Consult Beddard, "Earthworms and in Vol. II., cumbridge Natural History (New 1S96).