Reproductive system of Earthworm - Online Biology Notes

Reproduction in Earthworm

Earthworm are monoecious or hermaphrodite or bisexual.
However, self-fertilization doesn’t occur because of relative location of male and female reproductive organs as well as it is protandrous.
Protandrous refers the condition where male reproductive organs mature earlier than the female ones.
Thus, cross fertilization occurs in earthworm.
It comprises of male reproductive organs and female reproductive organs.

Male Reproductive Organs of Earthworm:

The male reproductive organs comprises of testes, testis sacs, seminal vesicles, spermiducal funnel, vasa deferentia, prostate glands and accessory glands.
The male reproductive organs include:

Structure	Number	Location
Testis	2 pairs	10^th and 11^th segment
Testis sacs	1 pair	10^th and 11^th segment
Seminal vesicles	2 pairs	11^th and 12^th segment
Spermiducal funnels	2 pairs	10^th and 11^th segment
Vasa differentia	2 pairs	10/11^th to 18^th segment
Prostate glands	1 pair	16/17^th to 20/21th segment
Accessory glands	2 pairs	17^th and 19^th segment
Common prosatic ducts	1 pair	18^th segment
Male genital aperture	1 pair	18^th segment

i. Testes:

Testes are very minute structures that are whitish in colour.
There are two pairs of testes and they are lobed.
One pair each is present in segment 10^th and 11^th that are found attached with the posterior surface of 9/10 and 10/11 inter-segmental septa.
Each testis possess 4-8 finger like lobules that contains rounded cells in masses termed as spermatogonia.
Testes are enclosed within testis sac.
During the young stage of the earthworm, the testes are fully developed whereas they get degenerated in the adult stage.

ii. Testis sacs:

Testis sacs are whitish, wide bilobed sacs that encloses 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.
In the testis sac, a large spermatic funnel having folded and ciliated margins is present behind each of the four testes.
The testis sacs links with a pair of seminal vesicles.
The spermatogonia are casted into the testis sacs then, they are 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 11^th and 12^th segment.
The testis sacs open into seminal vesicles by a narrow duct.
The posterior seminal vesicles are larger than the anterior ones.
The seminal vesicles of the 11^th segment are present enclosed in the posterior larger testis sac.
The seminal vesicles of the 12^th segment are exposed in the coelomic cavity.
These are located ventro-laterally below the stomach.
They are also termed as septal pouches, as they develop as septal outgrowths.
The spermatogonia from testis sac are received by seminal vesicles.
Seminal vesicles aids for nourishment to the sperm.
Here, the spermatogonia matures and form spermatozoa.

iv. Spermiducal funnel/ spermatic funnel:

They are cup like curvature in structure and are present in two pairs.
Each spermiduct funnel is internally ciliated.
It is located below each testis in the segment 10^th and 11^th segment.
It is found enclosed within the same testis sac.
It helps in conduction of spermatozoa.
After the maturation, sperms from seminal vesicles revert to testes sac and pass into vasa differentia via spermiducal funnel.

v. Vasa deferentia:

It is thin, long, narrow, thread like tubular structure.
Posteriorly, each spermiducal funnel leads to vasa deferens.
These are found in two pairs and each pair is located on the either side of the alimentary canal.
Both the pair of vasa deferentia runs in close proximity 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 prostatic duct in the 18^th segment forming common prostatic and spermatic duct.
The sperms from spermatic funnel are collected by the vasa deferentia and are supplied to prostate glands.

vi. Prostate glands:

Prostate glands are large, whitish, flat solid, 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 .
Maximum portion of prostate glands are glandular region while a small part is non-glandular region.
A thick curved prostatic duct arises from each prostate gland in 18^th segment.
The prostatic duct is connected to the two vasa deferntia of its own side and forms a 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 gland.
In earthworm the prostatic secretion is useful for the activation of sperms.
It also aids in motility of sperms.

vii. Accessory Glands:

Accessory glands are rounded structures and are found in two 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.
These glands are exposed to outside by a collection of small ductless glands.
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.
The secretion of these glands is thought to hold the two worms together during the copulation process.
After the completion of spermatogenesis, tailed spermatozoa are formed.
These spermatozoa again enter the testis sacs and reach the vasa deferentia via the spermatic funnels.
Then they are discharged through the male genital apertures along with the prostate glands’ secretion.
Male genital pores are found in pairs and is located in 18^th segment.

Female Reproductive Organs of Earthworm:

Female reproductive organs consist of the ovaries, oviducal funnel, oviducts and spermathecae.
Female reproductive organ includes:

Structure	Number	Location
Ovaries	1 pair	13^th segment
Oviducal funnels	1 pair	13^th segment
Oviducts	1 pair	13^th and 14^th segment
Female genital aperture	single	14^th segment
Spermathecae	4 pairs	6^th, 7^th, 8^th, and 9^th segments ventrolaterally

i. Ovaries:

Ovaries are white, small, lobulated structures that are found in a pair.
In the 13^th segment, ovaries are located one on either side of the ventral nerve cord.
It is attached with the posterior region of the inter-segmental septum of 12/13 segments.
Each ovary consists of various finger like projections where developing ova are found in a row, giving the beaded appearance.
In each ovarian lobe, the ova exhibits several stages of its development where mature ones lie in the distal part and the immature ones are found in the proximal part.

ii. Oviducts:

A small, ciliated ovarian funnel with folded margins are present below each ovary in 13^th segment.
Each ovarian funnel opens into a short and conical oviduct.
Oviducts are ciliated.
The oviducts of both the sides unite below the nerve cord.
Here, it open by a single median female genital pore ventrally in the 14^th segment.
The ova after maturation are released from the ovaries and are received by the ovarian funnel.
Then, it passes through the oviduct and that is discharged out via 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 made of ampulla and neck (upper short tubular part).
the main body is the ampulla.
At the junction of the ampulla and neck, a small sac is found.
This sac is termed as diverticulum.
Spermathecae are also termed as seminal receptacles as they are designed for receiving sperms from another worm during copulation and temporary storage of sperms.
The diverticulum of spermathecae in Pheretima posthuman stores sperms, which reach here after copulation.
But in other species of earthworm, sperms are store in ampulla.
In Pheretima posthuma ampulla aids nourishment to the sperm but in other species of earthworm, diverticulum provides nourishment.
Spermathecal duct opens outside by small pores situated in the grooves of 5/6, 6/7, 7/8, and 8/9 segments.

iv. Female genital pore:

It is single pore and is located in the 14^th segment.

Copulation in Earthworm:

Copulation has been seen in several species of earthworms.
It commonly take 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 in such a way that their ventral surfaces remain in touch and male genital openings of one comes just opposite to the spermathecal openings of the other and vice versa.
Both the earthworms receive sperms and prostatic secretion in all of their spermathecae by a protuberance that arises from the male genital opening.
Copulation lasts for about an hour.
Sperms are stored in the spermathecae.

Cocoon formation in Earthworm:

Fertilization only occurs in the cocoon or ootheca and is always external.
After maturation of ovaries, cocoon or ootheca formation starts after copulation.
Three varieties of glands are present in the epidermis of clitellar segments, i.e., 14^th , 15^th and 16^th segments.
They are mucous glands that secrete mucus for copulation, cocoon-secreting glands that produces the wall of the cocoon and albumen glands that secrete albumen in which eggs are deposited in the cocoon.
A cocoon of earthworm contains many fertilized eggs.
However, only one develops into embryo, while other fertilized eggs serve nutritive/ nurse cells.
Cocoon-secreting glands of the clitellum secretes a membranous girdle.
This girdle after hardening, the deposition of albumin between the girdle and the body wall takes place.
The worm begins to withdraw itself backwards from the girdle.
As the girdle moves over the female genital pore, it receives eggs, and when it passes over the spermathecae, sperms are emitted into it through spermathecal pores.
Finally, the girdle is removed off from the anterior end of the worm.
In a short time, the elasticity of its wall closes up its two ends forming a cocoon or ootheca.
Several cocoons formation occurs after each copulation as the spermatozoa present in the spermatheca do not move out all at one time.
The cocoons are oval in structure, light yellow in colour and are about 2 to 2.4 mm in length and 1.5 to 2 mm in breadth.

Fertilization in earthworm:

Fertilization occurs inside the cocoon and normally there is only one embryo in a cocoon.
Cocoon is found in moist and cool places and young one hatch out after 2-2.5 months.
Albumen cells provide nutrition to the embryo.
Cleavage is holoblastic and unequal.
Development is direct without any larval stage.

Summary points on Reproductive system of earthworm:

Earthworm are monoecious or hermaphrodite or bisexual and protandrous.
Each testis consists of 4-8 fingers like projections and are situated inside testis sac.
Each spermiduct funnel is internally ciliated and helps in conduction of spermatozoa.
Seminal vesicles in earthworm are also called septal pouches.
Seminal vesicles aids for nourishment to the sperm.
In earthworm the prostatic secretion is useful for the activation of sperms.
Each ovary is whitish in colour and consists of ovarian tubules.
Ovaries are larger than the testes.
Spermathecae are flask shaped structure and are found one pair in each 6^th, 7^th, 8^th and 9^th segments.
Each spermathecum is made of ampulla and neck (upper short tubular part).
At the junction of the ampulla and neck, a small sac is found termed as diverticulum.
This diverticulum of spermathecae in Pheretima posthuman stores sperms, which reach here after copulation.
But in other species of earthworm, sperms are store in ampulla.
In Pheretima posthuma ampulla aids nourishment to the sperm but in other species of earthworm, diverticulum provides nourishment.
Spermathecal duct opens outside by small pores situated in the grooves of 5/6, 6/7, 7/8, and 8/9 segments.
Spermathecae/ seminal receptacles are designed for receiving sperms from another worm during copulation and temporary storage of sperms.
It is assumed that, the secretion of accessory glands helps to keep the two earthworms together during copulation.
Copulation takes place between two earthworms generally at night during rainy season.
During copulation, two earthworms lie opposite to each other in such a way that their ventral surfaces remain in touch and male genital openings of one lies just opposite to the spermathecal openings of the other worm and vice versa.
Both the earthworms receive sperms and prostatic secretion in all of their spermathecae by a protuberance that arises from the male genital opening.
Sperms are stored in the spermathecae.
Fertilization only occurs in the cocoon or ootheca and is always external.
The glandular clitellum forms cocoons.
A cocoon of earthworm contains many fertilized eggs.
However, only one develops into embryo, while other fertilized eggs serve nutritive/ nurse cells.
Cocoon is found in moist and cool places and young one hatch out after 2-2.5 months.
Albumen cells provides nutrition to the embryo.
Cleavage is holoblastic and unequal.
Development is direct without any larval stage.