Explain the Reproductive System of Earthworm | NCERT Biology Class 12

Reproductive System of Earthworm | Class 12 Biology Note

Reproduction in Earthworm

Reproductive System of Earthworm- Earthworms are hermaphrodites, and cross-fertilization by copulation is the most common method of reproduction. The male genital apertures of one earthworm are close to the spermathecal pores of the other earthworm, and the earthworms are in contact in the opposite direction.

However, due to the relative location of the male and female reproductive organs, as well as it is protandrous, self-fertilization does not occur. Protandrous refers to the state at which male reproductive organs mature before the female reproductive organs. Thus, cross-fertilization takes place in the earthworms. It is made up of both male and female reproductive organs.

Reproductive Organs of Male Earthworm

The testes, testis sacs, seminal vesicles, spermiducal funnel, vasa-deferentia, prostate glands, and accessory glands are all parts of male reproductive organs.

The male reproductive organs include:

i. Testes:

• The testes are little entitled that are whitish in color.
• There exist two pairs of testes which are lobed.
• One pair each is present in segments 10^th and 11^th that are found attached with the posterior surface of 9/10 and 10/11 inter-segmental septa.
• Each testis possesses 4-8 fingers like lobules that contain rounded cells in masses termed as spermatogonia.
• Testes are enclosed within the testis sac.
• The earthworm’s testes are completely formed throughout the juvenile stage but degenerate during the adult stage.

ii. Testis sacs:

• Testis sacs are white, the bilobed sac that surrounds the testis.
• They are thin-walled and fluid-filled sacs.
• On the ventrolateral sides of the ventral nerve cord, the two testis sacs are located in the 10^th and 11^th segments beneath the stomach.
• Behind each of the four testes is a large spermatic funnel with folded and ciliated borders in the testis sac.
• A pair of seminal vesicles connects the testis sacs.
• Spermatogonia are thrown into the testis sacs and subsequently passed into the seminal vesicles.

iii. Seminal vesicles:

• These are large, whitish spherical structures.
• These are found in two pairs, each pair is located in the 11^th and 12^th segments.
• A thin channel connects the testis sacs to the seminal vesicles.
• The anterior seminal vesicles are larger than the posterior seminal vesicles.
• The seminal vesicles of the 11^th segment are presently enclosed in the posterior larger testis sac.
• In the coelomic cavity, the seminal vesicles of the 12^th segment are visible.
• These are located ventro-laterally below the stomach.
• They are also known as septal pouches because they form as septal outgrowths.
•  Seminal vesicles accept spermatogonia from testis sac.
• Seminal vesicles help to nourish the sperm.
• Here, the spermatogonia matures and form spermatozoa.

iv. Spermiducal funnel/ spermatic funnel:

• They have a cup-like curvature structure and exist in two pairs.
• Each sperm duct funnel is internally ciliated.
• It is located below each testis in the segment 10^th and 11^th segment.
• Within the same testis sac, it is found enclosed.
• It helps in conduction of spermatozoa.
• After maturity, sperms from seminal vesicles revert to testes sac and travel through spermiducal funnel into the vasa differentia.

v. Vasa-deferentia:

• It has a thin, long, narrow, tubular structure that looks like a thread.
• Each spermiducal funnel leads posterior to the vasa deferens.
• These are present in two pairs; one on each side of the alimentary canal.
• Both vasa-deferentia flow close together and laterally to the nerve cord below the alimentary canal on the ventral body wall.
• It extends from 12^th to 18^th segment and meets the prostatic duct in the 18^th segment forming the common prostatic and spermatic duct.
• The vasa-deferentia collect sperms from a spermatic funnel and deliver them to the prostate glands.

vi. Prostate glands:

• Prostate glands are huge, whitish, solid, and irregularly shaped glands.
• These are found in pair and are located one on either side of the alimentary canal in the segments from 16^th to 20^th or 17^th to 21^st.
• The majority of the prostate glands are glandular, with only a minor portion being non-glandular region.
• A thick curved prostatic duct arises from each prostate gland in 18^th segment.
• The prostatic duct joins the two vasa-deferentia of its own side to produce the common prostatic and spermatic duct.
• On the ventral side of 18^th segment, prostatic duct opens via a male genital aperture.
• Hence, each genital aperture has three distinct apertures, two of the vasa-deferentia and one of the prostatic glands.
• The prostatic secretion of earthworm aids in the activation of sperms.
• It also aids in motility of sperms.

vii. Accessory Glands:

• Accessory glands are spherical structures that come in pairs.
• Each pair is located in the segments 17^th and 18^th on the ventral body wall at the lateral sides of the nerve cord.
• A group of tiny ductless glands exposes these glands to outside.
•  On the two pairs of genital papillae positioned externally on either side of the mid-ventral line, these glands open in the 17^th and 19^th segments.
• These glands’ secretion is supposed to keep the two worms together throughout the copulation process.
• Tailed spermatozoa are created when spermatogenesis is complete.
• These spermatozoa enter the testis sacs once more and go to the vasa-deferentia through the spermatic funnels.
• They are then expelled through the male genital openings, coupled with the secretion of prostate glands.
• Male genital pores are found in pairs and is located in 18^th segment.

Reproductive Organs of Female Earthworm

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

The female reproductive organ includes:

i. Ovaries:

• Ovaries are whitish, lobulated, and tiny structures that come in pairs.
• Ovaries are found in the 13^th segment, one on each side of the ventral nerve cord.
• It is connected to the posterior part of the inter-segmental septum of segments 12/13.
• Each ovary is made up of a series of finger-like projections where developing ova are located in a row, providing the beaded look.
• The ova go through numerous phases of development in each ovarian lobe, with mature ones in the distal region and the immature ones in the proximal.

ii Oviducts:

• A small, ciliated ovarian funnel with folded margins is present below each ovary in 13^th segment.
• Each ovarian funnel gives way to a short and conical oviduct.
• Oviducts are ciliated.
• Below the nerve cord, the oviducts of both the sides unite.
• Here, it opens by a single median female genital pore ventrally in the 14^th segment.
• The ova are discharged from the ovaries during maturation and are received by the ovarian funnel.
• It then travels via the oviduct and is expelled by the female genital pore.

iii. Spermathecae:

• Spermathecae are four pairs and each pair is located ventro-laterally in the 6^th, 7^th, 8^th, and 9^th segments.
• Each spermatheca is flask shaped structure.
• Each spermathecum is formed by ampulla and neck (upper short tubular part).
• The main body is the ampulla.
• A tiny sac is present at the junction of the ampulla and the neck.
• This sac is termed as diverticulum.
• Spermathecae are also known as seminal receptacles because they are meant to receive sperms from another worm during copulation and to store sperms temporary.
• Sperms are stored in the diverticulum of spermathecae in Pheretima posthuman after copulation.
• Sperms on the other hand, are stored in ampulla of other earthworm species.
• Ampulla assists sperm feeding in Pheretima post-human, although diverticulum offers feeding in other earthworm species.
• Small apertures in the grooves of segments 5/6, 6/7, 7/8, and 8/9 allow the spermathecal duct to open to the outside.

iv. Female genital pore:

• It is a single pore and is situated in the 14^th segment.

Copulation

In several species of earthworms, copulation has been found.

• It commonly takes place in the rainy season during the months of July to October, in the morning hours before sunrise.
• During copulation, two earthworms lie opposite to each other with their ventral surfaces in contact, and male genital apertures of one come precisely opposite to the spermathecal openings of the other, and vice versa.
• A protuberance that originates from the male genital opening receives sperms and prostatic secretion in all of the earthworms’ spermathecae.
• Copulation lasts for about an hour.
• Sperms are stored in the spermathecae.

Cocoon formation

• Fertilization is always external and only occurs in the cocoon or ootheca.
• After the ovaries have matured, the creation of cocoon or ootheca begins.
• Three varieties of glands are present in the epidermis of clitellar segments, i.e., 14^th, 15^th and 16^th segments.
• Mucous glands release mucus for copulation; cocoon-secreting glands make the cocoon wall, and albumen glands release albumen into which eggs are placed in the cocoon.
• An earthworm’s cocoon comprises of many fertilized eggs.
• However, only one fertilized egg develops into an embryo, while the remaining fertilized eggs act as nutritive/ nursing cells.
• Cocoon-secreting glands of the clitellum secrete a membranous girdle.
• After hardening, the grindle deposition of albumin between the girdle and the body wall occurs.
• The worm starts to pull itself backwards from the girdle.
• The grindle accepts eggs as it travels over the female vaginal orifice, and sperms are discharged into it through spermathecal pores when it travels over the spermathecal.
• Finally, the girdle is detached from the worm’s anterior end.
• The elasticity of its wall quickly shuts up its two ends, producing a cocoon or ootheca.
• After the copulation, many cocoons develop because the spermatozoa in the spermathecal do not all migrate out at once.
• The cocoons are oval in shape, light yellow in color, and range in size from 2 to 2.4 mm in length and 1.5 to 2 mm in breadth.

Fertilization

Fertilization takes place inside the cocoon, and each cocoon usually contains just one embryo.

• Cocoons are found in damp and cold environments and the young one hatch after 2-2.5 months.
• Albumen cells provide nutrition to the embryo.
• Cleavage is holoblastic and unequal.
• Development is direct without any larval stage.

Important facts on Reproductive system of earthworm

• Earthworms are monoecious or hermaphrodite or bisexual and protandrous.
• Each testis comprises 4-8 fingers like projections and is located inside the testis sac.
• Each sperm duct funnel is ciliated on the inside and aids in the conduction of spermatozoa.
• Septal pouches are the other name for seminal vesicles in earthworms.
• Seminal vesicles help to nourish the sperm.
• The prostatic secretion in earthworms aids in the activation of sperms.
• Each ovary is whitish in color and consists of ovarian tubules.
• Ovaries are larger than the testes.
• Spermathecae are flask-shaped structures and are found one pair in each 6^th, 7^th, 8^th and 9^th segment.
• Each spermathecum is composed of ampulla and neck (upper short tubular part).
• A small sac is found termed as diverticulum is present at the junction of the ampulla and neck.
• This spermathecae diverticulum in Pheretima posthuman stores sperms that get here after copulation.
• Sperms, on the other hand, are stored in the ampulla of other earthworm species.
• Ampulla assists sperm feeding in Pheretima posthuman, although diverticulum offers nourishment in other earthworm species.
• Small apertures in the grooves of segments 5/6, 6/7, 7/8, and 8/9 allow the spermathecel duct to open to the outside.
• Spermathecae/ seminal receptacles are meant to receive sperms from another worm during copulation and to store the sperms temporarily.
• The secretion of accessory glands is thought to assist in holding the two earthworms together during copulation.
• Copulation occurs between two earthworms most often at night during the rainy season.
• During copulation, two earthworms lay opposite to each other in such a way that their ventral surfaces remain in contact, and male genital apertures of one worm are directly opposite to the spermathecal openings of the other worm and vice versa.
• A protuberance that originates from the male genital opening receives sperms and prostatic discharge in all of the earthworm’s spermathecae.
• Spermathecae are where sperms are kept.
• Fertilization takes place solely within the cocoon or ootheca and is always exterior.
• The glandular clitellum forms cocoons.
• A cocoon of earthworms comprises of many fertilized eggs.
• However, only one develops into an embryo, while the remaining fertilized eggs act as nutritive/ nurse cells.
• Cocoons are found in damp and cold environments and the young ones hatch after 2-2.5 months.
• Albumen cells provide nutrition to the embryo.
• Cleavage is holoblastic and unequal.
• Development is direct without any larval stage.

Takshila learning details about the reproductive system of the earthworm for the knowledge of the students of Biology. The mentors are there 24/7 for attending to the queries and solving the doubts of the aspirants.

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