Reproductive System of Earthworm

Earthworms reproduce sexually. They cannot reproduce asexually. They are hermaphrodite (monoecious) i.e. both ovaries and testes are found in the same individual. They are protandrous, the male sex cells mature much earlier than the female cells, therefore, self-fertilization is not possible. The relative position of ovary and testes also makes self-fertilization impossible. Cross-fertilization is preceded by copulation and cocoon formation.

Male Reproductive Organs:

The male reproductive organ comprises testes, testis sacs, seminal vesicle, vasa deferentia, Prostate gland and accessory glands.

Testes: The earthworm contains two pairs of testes, one pair in the 10th and the other pair in 11th segments found attached with the posterior surface of 9/10 and 10/11 intersegmental septa. They are found ventro-laterally beneath the alimentary canal on either side of the nerve cord.
They are minute, white, lobed structures made up of 4 to 8 finger like projections arising from a compact narrow base. The projections (processes) contain rounded, sperm mother cells or spermatogonia. The spermatogonia are shed into the testis-sac and pass into the seminal vesicles where they develop into spermatozoa. The testes are well developed only in young worms and become degenerated in adults.

Testis Sacs: Both the testes of each segment are found enclosed in a thin-walled, wide, bilobed and fluid-filled testis sac. These two testis sacs are found in 10th and 11th segments ventro-laterally one behind the other below the alimentary canal. These are coelomic space, cut off from the general body cavity that appear bilobed in front and enclose a pair of testis and a pair of ciliated spermatic funnels. The testis-sacs of the 11th segment are very large which enclose the seminal vesicles of its own side.

Spermiducal or spermatic funnels: There are two pairs of ciliated sperm funnels lying behind each testis, enclosed within the same testis sac.

Seminal Vesicles: Earthworm contain two pairs of large, white sac-like seminal vesicles lying in 11th and 12th segments respectively. They are also called septal pouches, as they are formed as outgrowths of the septa. The testis-sac of the 10th segment communicates with two seminal vesicles of the 11th segment and the testis-sac of the 11th segment communicates with two seminal vesicles of 12th segment. The posterior seminal vesicles are comparatively larger than the anterior ones. The seminal vesicles of the 11th segment remain enclosed in the posterior larger testis sac, while the seminal vesicles of 12th segment are exposed in the coelomic cavity.

Vasa Deferentia: Each spermiducal funnel leads into a slender, internally ciliated sperm duct or vas deferens. The two vasa deferentia of each side run close together along the ventral body wall on either side of the nerve cord from 12th to 18th segmenet. In 18th segment they join a thick prostatic duct.

Prostate glands: The prostate glands are a pair of large, white, flat, irregular and solid glandular masses lying one on either side of the gut and extending from 16th to 17th to 20th or 21st segment. Each gland consists of a large glandular part and a small non-glandular part. The glandular part is a racemose gland consisting of several lobes closely fixed together. The non-glandular portion consists of several small ductules. The ductules unite to form a short thick muscular, common curved prostatic duct in the 18th segment. The prostatic duct along with the two vasa deferentia of its own side are enclosed in a common muscular sheath but they remain separate. These three ducts are known as the common spermatic and prostatic duct, which open to the exterior independently by a pair of male genital pores ventrally on the 18th segment. The opening of the male genital pore is actually a group of three apertures, a large prostatic aperture and two small apertures of vasa deferentia. The prostate glands secrete the prostatic fluid of unknown function.

Accessory Glands: Earthworm contains two pairs of accessory glands, one pair in 17th segment and another pair in 19th segment on the ventral body wall at the lateral sides of the nerve cord. They are rounded, white fluffy masses which open to the exterior by a number of ducts on two pairs of genital papillae, situated mid-ventrally in the 17th and 19th segments. Secretion of accessory glands probably helps in uniting the two worms during copulation. The mature spermatozoa from the seminal vesicles move back to the testis sac and pass through the sperm funnels, the vasa deferentia and finally discharged through the male genital pore with secretion of prostrate glands during copulation.

Female Reproductive Organs:

The female reproductive organs are ovaries, oviducal funnels, oviducts and spermathecae.

Ovaries: A pair of ovaries remain attached to the posterior face of septum 12/13 one on either side of the nerve cord. They are found in 13th segment. Each ovary is a white compact mass made up of several finger-like processes. The processes contain ova in various stages of development arranged in a linear series.

Oviducal Funnels: In the 13th segment a large saucer-shaped oviducal funnel lies below each ovary. Each funnel has folded and ciliated margin, which leads into a short oviduct.

Oviducts: The oviducts are two short conical ciliated tubes. The oviduct run posteriorly and converge to meet below the nerve cord forming a very short common oviduct. It opens to the outside by a single median and ventral female genital aperture in the 14th segment.

Spermathecae: There are four pairs of flask-shaped spermathecae situated in sixth, seventh, eighth and nineth segments. Each spematheca has a body known as the ampulla. The ampulla continues into a short narrow neck, which gives off a simple narrow, blind and elongated diverticulum or caecum. The diverticulum serves for the storage of sperms received from another worm during copulation. The ampullae provide nourishment to the stored sperms. The spermathecae open exterior by four pairs of separate spermathecal pores lying ventro-laterally in the grooves between 5/6, 6/7, 7/8 and 8/9 segments. Eggs come out of the ovaries and are entangled by the oviducal funnels, pass through the oviducts and finally pass out through the female genital pore to be laid inside the cocoon.

Copulation:

It usually occurs in night or early morning during the rainy season and lasts or about one hour. During copulation two earthworms become closely applied to each other by their ventral surfaces with the anterior ends pointing in opposite directions. In this position the male genital apertures of each worm lie opposite to the spermathecal pores of the other. the areas of the male genital apertures are raised into papillae and are inserted successively into the various pairs of spermathecal pores from behind to forward, so that a mutual exchange of prostatic fluid and spermatozoa occurs between the two copulating worms. Copulation lasts for about one hour and the spermatozoa and prostatic fluid are stored in spermathecae. After the completion of this process the worms separate.

Cocoon Formation:

After copulation, formation of cocoon starts when the ovaries mature. The cocoon-secreting glands of clitellar segments secrete a viscid or gelatinous substance forming a broad membranous band or girdle around the clitellum. The girdle hardens gradually on exposure to the air into a tough but elastic tube, which becomes the cocoon or egg capsule. The albumen glands produce albumen in which eggs are deposited in the cocoon. The worm withdraws itself backwards from the girdle and the cocoon slips forward towards the anterior end. On its way the cocoon receives ova as it passes over the female genital aperture and sperms when it passes over the spermathecal pores. As the cocoon is thrown off from the anterior end of the worm, its both ends close up owing to the elasticity of its walls. The cocoons are more or less oval or spherical and light yellow in colour. fertilization is external. It takes place inside the cocoon. Each ovum is fertilized, but as a rule only one fertilized egg develops into embryo and the rest degenerate. the albumen stored in the cocoon serves for the nourishment of the growing embryo. Cocoons are laid from August to October in damp situations. Many cocoons are formed after each mating as the sperms stored in spermathecae are not pass out at once.

Development:

Development of earthworm occurs within the cocoon. The developing embryo is nourished by other fertilized eggs and the albumen stored in the cocoon. The development is direct without any larval stage. The zygote divides by holoblastic and spiral cleavage resulting in the formation of a hollow ball of cells, the blastula. The blastula remains enclosed into a vitelline membrane. The blastula contains upper ectodermal cells and a layer of lower ectodermal . Later a gastrula is formed by invagination of endodermal cells into ectodermal cells. The gastrula contains an archenteron cavity which open exterior through a wide aperture, the blastopore. The blastopore later reduced to become the mouth. The mesoderm develops from two cells of blastula, called mesoblasts. They divide to form two mesoblastic bands which later give rise to the coelomic epithelial lining. The embryo elongates, the mesoderm band extends between the ectoderm and endoderm and gets divided into segments. Each segment of the mesoderm acquires a coelomic cavity. the ectodermal cells form 3 to 4 rows of cells behind a large cell known as teloblast. The innermost ectodermal row forms the nerve cord. The nerve cord forming cells are known as neuroblast cells. The next two rows of cells (nephroblast) from the nephridia and setal sacs. The mouth and anus opens into the endodermal enteron to form the alimentary canal. The embrto hatches from the cocoon as a small worm, which resembles the adult worm except for size and absence of clitellum.

Regeneration:

Earthworms have ability to regenerate. A new head may be regenerated after the removal of up to fifteen anterior segments. The posterior segments are more easily regenerated. If a cut is made near the tail end, another tail develops from the cut surface.

Economic Importance:

Earthworms are of great economic importance to man. They play an important role in agriculture and are the best friends of farmers as they are continously ploughing and manuring the soil. However, a few species are harmful.

Earthworms in Agriculture: Earthworms make burrows and hence aerate the soil. They continually bring the lower soil and deposit it on the surface. it is said that in ten years time, they can fill the whole surface of earth up to 5 cm. The earthworms have been ploughing the land long before man. Their habit of burrowing and soil feeding makes the soil loose and porous. Their burrows permit the penetration of air and moisture in the porous soil, improve drainage and make the downward growth of roots easier. The faeces of the worms make a good manure. By burying the dead and decayed matter, they enrich the soil. The excretory wastes and other secretions of the worms also enrich the soil by adding nitrogenous matters that form important plant food. The faeces of earthworm contain nitrate, calcium, phosphorus, potassium and magnesium which constitute an important component of the humus essential for plant growth. They also reduce the alkalinity and acidity of the soil.

Earthworm as Bait and Food: Earthworms are used as baits foe fishing in many parts of the world. They are used as food by large number of animals like frogs, moles, lizards, small snakes, centipedes and also some birds. In many parts of the world uncivilized people use the earthworms as food. The earthworms make the best food of fish aquarium and small animals in laboratory.

Earthworms in Medicines: Ayurvedic and Unani system of therapy suggests that earthworms are used for reducing the size and finally removing bladder stones, jaundice, piles, diarrhoea, weakness after pregnancy, sexual impotency and gout. Today they are used in India, China and Japan in various fancy medicines.

Earthworms in laboratories: Earthworms are universally used for dissection in zoological laboratories, as they are easily obtained.

Harmful Earthworms: A few species of earthworms cause harm to man in various ways:

1) Their burrows may cause loss of water by seepage from ditches, canals and irrigated lands. Their casting on slopy lands easily get washed away by rain and thus contribute to soil erosion.

2) Some species bury in the carcasses of animals, and bring disease germs to the surface, which infect others. While a few species live as ectoparasite on frogs and man. They also act as intermediate host of some parasites like the tapeworm (Amoebotaenia sphenoides) of chicken and the lung nematode (Metastrongylus elongatus) of pig. Pheretima elongata damages the roots of betel vine, Malabaria podudicola and Aphanascus oryzivorus damage roots of paddy.