Animals are heterotrophs and depend directly or indirectly for their nutrition on plants, photosynthetic algae or bacteria. Much of the diversity of animals evolved as they acquired the ability to capture and devour different types of food.
Earthworm belongs to the Phylum Annelida, Genus Pheretima and Species posthuma. There are about 500 species of Pheretima and 13 of them are found in India.
Earthworms are terrestrial animals and inhabit the moist soil. These are nocturnal animals and generally live in the upper layer of soil to a depth of about 30 to 45 cm. During daytime, they live in underground burrows; they make burrows partly by boring and partly by swallowing the soil. In the gardens, they can be traced by their faecal deposits known as wormcasts.
Earthworms are reddish brown in colour and the dorsal surface is darker than the ventral surface. The body of the earthworm is cylindrical, elongated and pointed anteriorly. The posterior end is slightly rounded. A fully-grown worm measures about 150 mm in length and 3 to 5 mm in width. The ventral surface is distinguished by the presence of general openings (pores). The dorsal surface of the body is marked by a dark median line (dorsal blood vessel) extending along the length of the body. The earthworm lacks a distinct head.
The entire body is made up of a series of distinct segments or metameres, which are separated from each other by intersegmental grooves.
The number of segments varies from 100-120. These segments are internally divided by septa. The four anterior segments in Pheretima posthuma show external segmentation but their corresponding internal septa are absent. The first segment at the anterior end of the body is called the `buccal segment' or peristomium, which bears a very small terminal opening of the mouth. A small projection is also present which hangs over the crescent shaped mouth and is called prostomium. The anus is located at the posterior end of the last segment. In a mature worm, a prominent circular band of glandular tissue surrounds segments from 14 to 16. These fused segments are designated as clitellum. Thus, the body is divisible into preclitellar, clitellar and postclitellar segements. Spermathecal apertures are four pairs and are situated at the ventro-lateral sides of the intersegmental grooves between 5 and 6,6 and 7,7 and 8 and 8 and 9 segments. The single female genital pore is present in the mid-ventral line of the 14th segment. A pair of male genital pores is present on the ventro-lateral sides of the 18th segment. Numerous minute pores called nephridiopores, which are openings of the nephridia (excretory organs), open on the ventral surface of the body.
In all the body segments, except the first, last and clitellum, there is a ring of S-shaped setae, made up of chitin that helps in locomotion by gripping the earth. These are not visible to the naked eyes.
The body wall of earthworm is covered externally by a thin - noncellular cuticle. The outermost cuticle is followed by an epidermis, two muscle layers and an innermost coelomic epithelium. The epidermis is made up of a single layer of columnar epithelial cells and contains many other types of cells, including the secretory gland cells. The muscle layers are composed of circular and longitudinal muscle fibres.
The excretory organs occur as segmentally arranged coiled tubules called nephridia (sing,Nephridium). These are of three types
- Those present on both the sides of intersegmental septa are called septal nephridia. These open into intestine.
- Those found attached to the lining of the body wall are called integumentary nephridia. These open on the body surface.
- Those found in the 4th, 5th and 6th segments in the form of three paired tufts are called pharyngeal nephridia. These nephridia exhibit a basic similarity in their structures.
Pheretima represents a closed type of blood vascular system comprising blood vessels, capillaries, and heart with valves. Blood glands, aggregates of follicles, are present on the 4th, 5th and 6th segments. These produce blood cells and haemoglobin. Haemoglobin remains dissolved in blood plasma. Blood cells do not possess haemoglobin. These are phagocytic in nature.
The nervous system is represented by ganglia arranged segment-wise on a ventral nerve cord. A ganglion is a mass of nerve cells. The nerve cord in the anterior region (3rd and 4th segments) bifurcates, laterally encircling the oesophagus and uniting dorsally in the form of a nerve ring. The nerve ring with cerebral ganglia represents the brain. Sense organs are represented by epidermal, buccal and photoreceptors, and are poorly developed.
Earthworm is hermaphrodite or bisexual i.e. testes and ovaries are present in the same individual.
There are two pairs of testes present in the 10th and 11th segments. Their ducts, i.e. vas deferens, run up to the 18th segment where they join the prostatic duct. Accessory glands are present on the ventral side of the 17th and 19th segments. The common prostate and spermatic duct opens to the exterior by a pair of male genital pores on the ventro-lateral side of the 18th segment. Four pairs of sac-like structures called spermathecae are found one in each of the 6th to 9th segments. They receive and store spermatozoa during copulation. One pair of ovaries is attached at the intersegmental septum of the 12th and 13th segments. Ovarian funnels are present beneath the ovaries, which continue into oviduct, join together and open on the ventral side as a single female genital pore on the 14th segment. Development is direct, i.e. there are no larval forms.
Fig: Internal Morphology of Earthworm
Interaction with Mankind
Earthworm makes small burrows and the soil becomes porous, facilitating respiration and penetration of developing plant roots. Hence they are known as `friend of farmers'. The decaying vegetation, which is taken by earthworm to burrows, enriches the soil. Soil particles are also broken to a fine state by the muscular gizzard. The digested and decayed vegetation and fine soil is deposited as worm casting. The nitrogenous wastes added to the soil are also used by plants. The process of increasing fertility of soil by earthworms is called vermicomposting. Earthworms are also used as food in some countries such as China, Japan, Myanmar (Burma), Australia, etc. Many tribal communities in India use earthworms in the form of medicine to cure bladder stones, jaundice, piles, diarrhoea, etc. Earthworm is also used as a bait for game fishing. Earthworms may cause some harm to man. They may damage young and tender plants. Besides, they also damage land by making burrows. This may cause seepage in areas where water table is high causing erosion of soil at slopes. The population of earthworm is declining due to uncontrolled pesticidal sprays and chemical fertilisers. Many animals such as frogs, birds and lizards, which prey upon them are also being affected indirectly resulting in ecological imbalance.