Reproductive System of Pheretima (Earthworm)!
Contents
Male Reproductive Organs of Earthworm:
The male reproductive organs consist of testes, testis sacs, seminal vesicles, vasa deferentia, prostate glands and accessory glands.
Testes and Testis Sacs:
The testes are two pairs, very minute, whitish and lobed structures situated one pair each in segment 10th and 11th found attached with the posterior surface of 9/10 and 10/11 inter-segmental septa. Each testis consists of 4 to 8 finger-like lobules projecting from a compact base.
Each lobule of the testis contains rounded cells in masses called spermatogonia. The testes are well developed only during the young stage of the worm and become degenerated in the adult worm. The testes of each segment are found enclosed in a thin-walled, wide bilobed and fluid-filled sacs called testis sacs.
Thus, there are two testis sacs situated in the segments 10th and 11th on the ventrolateral sides of the ventral nerve cord beneath the stomach. \
Behind each of the four testes, in the testis sacs, is a large spermatic funnel having folded and ciliated margins. The testis sacs remain in communication with the seminal vesicles. The spermatogonia are shed into the testis sacs and pass on into the seminal vesicles where they undergo maturation and form spermatozoa.
Seminal Vesicles:
These are two pairs of large, white, sac-like bodies, one pair each in the segments 11th and 12th into which the testis sacs open by narrow ducts. The anterior seminal vesicles are smaller than the posterior ones. The seminal vesicles of the 11th segment are found enclosed in the posterior larger testis sac, while those of the 12th segment are exposed in the coeiomic cavity.
These are also placed ventro-laterally beneath the stomach and since they develop as septal outgrowths, hence, they are also known as septal pouches.
Vasa Deferentia:
Each spermatic funnel (posteriorly) leads into a thin, narrow, thread-like sperm duct or vas deferens. Thus, the two vasa deferentia of each side run very closely and laterally to the nerve cord beneath the alimentary canal on the ventral body wall, up to 18th segment where they join the prostatic duct.
Prostate Glands:
These are a pair of large, white, solid and irregularly- shaped glandular masses situated one on either side of the gut in the segments from 16th or 17th to 20th or 21st. Each prostate gland consists of a maximum glandular region and a small part of non-glandular region. From each prostate gland emerges a thick curved prostatic duct in 18th segment.
This duct joins the two vasa deferentia of its side and these three ducts are enclosed in a common sheath to form a common prostatic spermatic duct which opens separately through a male genital aperture on the ventral side of the 18th segment.
Thus, each genital aperture has three separate apertures—two of the vasa deferentia and one of the prostatic gland. The function of the secretion of prostatic duct is not definitely known.
Accessory Glands:
These are two pairs of rounded structures situated one pair each in the segments 17th and 18th on the ventral body wall at the lateral sides of the nerve cord.
These glands open to the outside by a number of small ductless on the two pairs of genital papillae situated on the either side of the mid-ventral line in 17th and 19th segments externally. The secretion of these glands is believed to keep together the two worms during the process of copulation.
As mentioned earlier, the spermatogonia from the testes are shed into the testis sacs and pass on to the seminal vesicles where spermatogenesis is completed and tailed spermatozoa are formed. Now, these spermatozoa again enter the testis sacs and through the spermatic funnels go down the vasa deferentia and are discharged through the male genital apertures along with the secretion of prostate glands.
Female Reproductive Organs of Earthworm:
Female reproductive organs consist of the ovaries, oviducts and spermathecae.
Ovaries:
There is a pair of white, small, lobulated ovaries, one on either side of the ventral nerve cord in the 13th segment attached with the posterior face of the inter-segmental septum of 12/13 segments.
Each ovary has several finger-like processes with developing ova in a row providing it beaded appearance. The ova remain in various stages of their development in each ovarian lobe being mature in the distal part and immature in the proximal part.
Oviducts:
Below each ovary in 13th segment, there is a small ovarian funnel with folded and ciliated margins. Each ovarian funnel leads into a short, conical and ciliated oviduct.
The oviducts of both the sides converge to meet below the nerve cord and open by a single median female genital aperture ventrally in the 14th segment. The mature ova are released from the ovaries and received by the ovarian funnel, pass through the oviducts and which go out through the female genital aperture.
Spermathecae:
These are four pairs, one pair each in the segments 6th, 7th, 8th, and 9th situated ventro-laterally. Each spermatheca is flask-shaped, the main body is the ampulla which is continued into narrow duct— the neck with a small diverticulum on its inner side. These are also called seminal receptacles as they store the spermatozoa from the another worm during copulation.
In Pheretima, unlike the other earthworms, the diverticula store the spermatozoa and not the ampulla. The spermatheca open externally by their small ducts as spermatheca pores situated inter-segmentally between segments 5/6, 6/7, 7/8 and 8/9.
Copulation, Cocoon Formation and Fertilisation of Earthworm:
Copulation has been observed in different species of earthworms. It usually occurs in the rainy season during the months of July to October at dawn, i.e., in the morning hours before sunrise.
During copulation two earthworms of adjacent burrows half emerge from their burrows and come closer to lie in contact to each other by their ventral surfaces with their anterior ends pointing in opposite directions. In this position the male genital apertures of each worm lie opposite the spermathecal pores of the other.
The areas of the male genital apertures are raised into papillae and inserted into the spermathecal pores, so that there is a mutual exchange of spermatozoa and prostatic fluid between the two copulating worms. Copulation lasts for about an hour or so, then the worms separate and recede into their burrows.
Cocoon or ootheca formation starts after copulation when ovaries mature. The epidermis of clitellar segments, i.e., 14th, 15th and 16th segments contain three kinds of glands, they are unicellular mucous glands which produce mucus for copulation, cocoon-secreting glands secrete the wall of the cocoon and albumen glands produce albumen in which eggs are deposited in the cocoon.
A membranous girdle is secreted by cocoon-secreting glands of the clitellum, this girdle soon hardens, then albumen is deposited between the girdle and the body wall. The worm starts withdrawing itself backwards from the girdle. As the girdle passes over the female genital aperture it receives eggs, and when it passes over the spermathecae sperms are extruded into it through spermathecal pores.
Lastly, the girdle is thrown off from the anterior end of the worm and soon the elasticity of its wall closes up its two ends to form a cocoon or ootheca. Several cocoons are formed after each copulation because the spermatozoa contained in the spermatheca do not pass out all at one time. The cocoons are oval, light yellow in colour and are about 2 to 2.4 mm long and 1.5 to 2 mm broad.
Fertilisation takes place inside the cocoon and generally there is only one embryo in a cocoon. Cocoons are laid from August to October in damp situations.