Earthworm

Earthworm. The common earthworm, Lumbricus terrestris, is a very convenient type to serve as an introduction to the study of the worms, as its anatomical structure is fairly easily made out by dissection, and it is so extremely common. It belongs to the class Oligochaeta (q.v.). When examined externally, the earthworm is seen to consist of a long cylindrical body, divided into from 100 to 200 cylindrical rings or segments by means of well-marked furrows. The mouth opens on the second of these rings, and the segment in front of it is a small conical point known as the "prostomium"; the anus is situated at the extreme posterior end. The body tapers sharply to a point at the anterior end and more gradually posteriorly; the rest of the body is of a uniform diameter except for segments numbers 31 to 38, counting. from the front end; these are expanded by a number of glands into a region known as the "clitellum," the function of which is the formation of the cocoon by which the eggs are united into masses. Running along each side of the ventral aspect of the worm is a pair of faint raised lines; on each of these during its passage across a segment a pair of small bristles or setae occur; these are very fine, and are often lost; they represent the strong conspicuous setae of the Polychaeta (q.v.). If the earthworm be cut open, the dorsal side of the body cavity will be found to be divided into a number of separate chambers by vertical partitions or dissepiments, which correspond in position to the depressions which mark off the somites. The digestive system consists of one long tube running straight from the mouth to the anus; immediately behind the mouth it expands out into a large "pharynx," occurring in the first five somites; this is continued as a narrow tube, the oesophagus, which dilates into the crop in the 16th and 17th somites; behind this occurs the gizzard, and then comes the long straight simple intestine; as this is straight, instead of convoluted, additional internal digestive and absorbent surface is obtained by an infolding of the upper side of the wall, known as the "typhlosole." There is no distinct heart in the earthworm, but there is a complex series of blood vessels. The largest of these is the "dorsal," and runs along the upper side of the alimentary canal; it is connected by numerous lateral vessels with smaller longitudinal canals on the under side. The nervous system consists of a distinct "ganglion" above the mouth; a pair of lateral nerves pass from this one on each side of the pharynx, and unite below in a pair of ganglia in the second somite; thence a long cord passes backward through the length of the worm on the under side; in each segment it expands out into a distinct ganglion. The renal organs or "nephridia" occur as a pair in each somite in all except the first three. One end is a funnel, on the margin of which occur long lashing cilia; this funnel floats freely in the fluid which fills the body chambers; it leads into a long narrow tube which passes through the diaphragm between the two somites, and opens to the exterior in the next posterior somite. The reproductive organs are included in the 10th to 15th somites; the earthworms are hermaphrodite, male and female organs occurring in the same individual. Organs of special sense are very imperfectly developed; earthworms are blind, but can distinguish between light and darkness; they are also completely deaf, and have but a feeble sense of smell. Their sense of touch is, however, very well developed. Earthworms live, as a rule, burrowing through mould and soil containing much decaying vegetable matter. They carry leaves into their burrows, prepare these for food by covering them with a saliva, which partly digests them before they are actually eaten (extra-stomachal digestion). The main food of the earthworms is, however, obtained from the soil through which they burrow; the earth removed is swallowed, the organic matter in it is absorbed, and the excreta discharged, forming the casts found about the entrances to worm burrows. The earthworm thus constantly raises parts of the soil to the surface; in suitable soil they exist in great numbers. Hensen calculated the number at over 50,000 an acre. The influence of worms in rearranging the soil is very considerable; as has been observed, they carry down leaves to line their burrows, and thus act as fertilising agents; by gradually removing the soil from under stones, etc, and piling it up around them, they ultimately bury and preserve them. The part played by earthworms is thus an exceedingly important one. In some tropical districts where the soil is unsuited for worms, the white ants (Termites) play their part.