An overview Earthworm is a terrestrial worm inhabiting mostly on top layers of the moist soil. It is one of the farmer-friendly organisms usually thin and reddish-brown. They form the fecal deposits called worm castings ;a highly nutritive choice for crops. In India, 2 of the most common earthworm species are Pheretima and Lumbricus although there are 13 other well-known species. Worldwide, there are 5,500 named species of earthworms. This post explains the classification, positioning, structure and various other features of earthworms.
Systematic Position of earthworms
Features of earthworms
The adult worm measures around 150 mm long and 3-5 mm wide with their cylindrical body throughout. The dorsal surface looks darker than the ventral surface.
Locomotion, nutrition and reproduction
Earthworm moves slowly by crawling (creeping) and their movement is supported by muscular contraction and relaxation of the trunk with the help of chitinous setae or chaetae. They prey upon decaying organic matter found in the moist topsoil. The food is digested in the gut and undigested food along with the soil forms worm castings. Earthworm is monoecious (having only one type of sex organ). They essentially breed throughout the rainy season. They are protandrous in nature (i.e., male sex organs mature earlier than the female) furthermore, there is no chance for self-fertilization. It has a great capacity to regenerate by itself even if it is cut into many large segments. When it is cut, the anterior half of the worm develops into the tail.
The body of an earthworm is very well distinguished because of the segmentation. This helps its body to be subdivided into more than a hundred short segments and the process is known as metamerism. The mouth is located in the first segment called peristomium. There are prominent circular bands of muscles present on the 14th, 15th and 16th segments called the clitellum. It has a single genital pore at 14th segment which is placed mid-ventrally. A pair of male genital pores are present on the ventral surface of the 18th segment.
The body of earthworm is made up of 4 layers ; cuticle, epidermis, musculature and coelomic epithelium. The outer surface is a thin, non-cellular layer called the cuticle. The epidermis contains a single layer of columnar epithelium along with secretory gland cells. Muscle fibres are of 2 types with an outer thin layer of circular muscle fibers and an inner layer of longitudinal and thick muscle fibers. The outermost coelomic epithelium consisting of flattened squamous cells help in protecting the internal organs.
Earth worm`s alimentary canal is segmented through the entire length beginning from the mouth and ends with the last segment called anal opening. The alimentary canal is divided into the buccal cavity (first to third segments), Pharynx as the 4th segment and oesophagus lies within the segment 5 to -7. The area between the segments 8-9 is called Gizzard. The stomach is found lying between 10th and 14th segments, the intestine is formed at the segment 15. From its segment 16 it continues to form the anus. Earthworm`s skin aids in respiratory process called cutaneous respiration. The outer surface is thin, transparent and richly supplied with enormous blood vessels. Like many other animals, earthworms have a closed vascular system made up of heart, blood vessels, and tiny capillaries. The blood flows from posterior end to the anterior end of the body. The worm has 4 pairs of hearts found at 7th, 9th, 12th and 13th segments respectively. The excretory organs look like coiled tubes arranged in a segmental fashion. They lie near the inter-segmental septa of segment 15. The worms have a nervous system made up ganglia arranged in the form of segments around the ventrally found paired nerve cord.
Importance of Earthworms Roundworms are farmer-friendly because of their faecal casts. Casts are formed as non-digested wastes, and they are rich in nitrogen. The method of farming by using the worm casts is called vermicomposting. They are also used while catching fish. Countries like China and Japan use them as a part of their routine food.