Earthworms

a general name for several families of annelid worms of the class Oligochaeta.

The earthworm body is composed of rings, or segments (from 80 to 300). Each segment except the front one has eight (in some tropical species, several dozen) short setae, which provide support for crawling. The body length of large tropical earthworms is up to 2 m; those found in the USSR are 8-15 cm long, rarely reaching 40 cm. Colors range from light-gray to blackish-brown or cherry red. Earthworms have a well-developed circulatory system with red blood; respiration is cutaneous. The mucus produced by skin cells protects the body from drying and mechanical damage. The nervous system consists of a poorly developed brain and a ventral nerve cord. There are no sense organs, but the skin has many light-sensitive cells.

Earthworms are hermaphroditic. They reproduce by forming cocoons inside which fertilization and development of eggs take place. The cocoon is secreted by the clitellum, a glandular thickening of the skin in several anterior segments (in most species found in the USSR, between the 25th and the 40th segments). In two to four weeks little earthworms emerge from the cocoon; they reach adult size within three or four months. Earthworms have a highly developed power of regeneration. They live in soil where they move by pushing aside soil particles with their heads or by swallowing them. Their burrows penetrate to depths of no less than 60-80 cm, and large species may burrow to depths of 8 m. Earthworms lead a nocturnal life, appearing above ground in the daytime only after heavy rains (hence, the Russian name of “rainworms”), when they begin to suffocate owing to lack of oxygen in the water-saturated soil. Earthworms feed on a variety of plant residues, manure, and so forth. Excrement deposited on the surface by earthworms in large amounts has the form of characteristic small heaps of soil.

Altogether there are about 1,500 species of earthworms. Most live in the tropics. There are approximately 100 species in the USSR. As C. Darwin explained, earthworms are of great importance in soil formation. They loosen the soil by digging burrows, thus aiding aeration and permitting moisture to penetrate. They mix the earth and hasten decomposition of plant residue. However, earthworms at the same time cause some damage, since they are intermediate hosts of pulmonary helminths of swine and certain parasites of poultry (such as of chickens and ducks).