I doubt whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures. ―Charles Darwin on the importance of earthworms.
The earthworm plays a major role in the proper functioning of the ecosystem of the soil. It acts as a scavenger, and helps in the recycling of the dead and decayed plant matter by feeding on it. It increases the soil fertility and is often referred to as a farmer's friend. It burrows the soil and ingests soil particles coming in its way. Both these processes aerate the soil and help in the inter-mixing of the soil particles of the upper and underlying layers.
Earthworms are found all over the world in any type of soil, except the waterlogged and sandy areas. Let's discuss in brief about the taxonomic classification and characteristics of earthworms.
Taxonomy of the Common Earthworms
Though all species of earthworm are classified in the same class and order, they do not belong to the same family. Following is the classification of the common earthworm (Lumbricus terrestris).
Earthworms are eukaryotic (cells have nuclei), multicellular organisms. They have the ability to move and depend on dead plant materials and microorganisms for food.
Earthworms belong to the phylum annelida which comprises segmented worms. The segments of the earthworm's body, known as annuli, are separated by transverse dividing walls known a septa. They have multiple segments, with those belonging to a species possess organs in same segments. In some species of annelids, septa are less defined or even absent.
Earthworms have clitellum, a type of collar that secretes clitella or cocoon during reproduction. The head of earthworm is less developed than other annelid species.
Earthworms have setae or bristles on the body, which helps them to attach to the surface during movement. They lack lateral appendages or parapodia, which is a characteristic feature of the subclass polychaeta.
The common earthworm is categorized under Haplotaxida, which is one of the two orders of Oligochaeta.
The common earthworm belongs to Lumbricidae which is one of the largest earthworm family. About 33 species of earthworms are identified under this family.
Characteristics of the Common Earthworm
The common earthworm (Lumbricus terrestris) resembles a cylindrical tube, with an average length of about 7 cm. - 8 cm., with some members of this species even growing to 35 cm. They are found abundantly in North America, Europe and western Asia.
The reddish-gray colored body of the earthworm is segmented, and the vital organs are present in particular segments. The skin is covered by a moist mucous layer that serves the main purpose of respiration (exchange of air). An earthworm does not have any locomotory organs and therefore moves by means of muscle contraction and relaxation.
The earthworms are also known as night crawlers because they are usually come above ground during the night. During the day, they burrow the ground using their strong toothless yet muscular mouths. While burrowing, an earthworm feeds on dead plant materials and organic matter present in the soil. The ingested food is broken down into finer particles in its muscular stomach also known as gizzard. The fine food particles are acted upon by various enzymes for digestion process. Useful nutrients are absorbed and undigested soil and other particles are passed out as worm casts. Studies have revealed the presence of useful soil microorganisms in earthworm casts.
The earthworms also transfer nutrients and minerals from the earthen layers below, to the surface above through their waste. The small burrows that they create keeps the soil aerated. Thus the earthworms play a vital in maintaining the health of the soil.
Earthworms are hermaphrodite, meaning both male and female sex organs are present in the same body. However, reproduction takes place via cross-fertilization. The eggs are enclosed in an egg casing or a cocoon. The juvenile earthworm resembles an adult worm, except that it lacks sex organs. It attains sexual maturity within 2 - 3 months after hatching.
One of the characteristic features of many different species of earthworms is their ability to regenerate lost segments of their bodies. The lifespan of the earthworm varies depending upon the species; the common earthworm can live up to 6 years in the wild. Common predators of the earthworm include birds and other small mammals.