The biological process in which new individuals are produced from the parents is called reproduction. The process of reproduction is required to carry on the generation of a species. If the process of reproduction would not have existed, then almost all of the species that we know would have become extinct. These offspring have characteristics similar to that of their parents and can also exhibit some new traits. Reproduction is a very crucial concept for a species to exist on earth. Each and every one of us is a product of reproduction. Reproduction is essential not only for keeping a species from vanishing from the surface of the earth, but it also leads to evolution. Through the years, as organisms keep reproducing, we are aware that they pass on their traits to the younger generation. Broadly speaking, reproduction is of two types: sexual and asexual. In sexual reproduction, two parents are required whereas, in asexual reproduction, a single parent is required. Some organisms reproduce sexually whereas some adopt the asexual method of reproduction.
One peculiar organism which has both the male and female sexual organs are earthworms. Earthworms belong to the phylum Annelida in the Animal Kingdom. They are tubular in shape and are segmented in a series called metamerism. These segments divide the body of an earthworm into different compartments. These segments comprise of different organs required for the formation and functioning of an earthworm. Earthworms can even regenerate the segments which they lose but it will depend on the severity of damage in its segments.
Earthworms are hermaphrodites, meaning, they consist of both the male and female sex organs. These sexual organs are often located in the segments nine to fifteen. Usually, they mate at night and even though they prefer living underground, the process of reproduction takes place at the surface. The most important sex organ in an earthworm is an organ named clitellum. The clitellum is located in the clitellar region of the body. The clitellar region has setae (which looks like small hairs sticking out of the skin of the earthworm) when the worms at an immature state but before the formation of clitellum, setae are shed off. The clitellar region consists of various other cells namely, mucous cells, the cells which are responsible for secreting mucus that helps in the formation of cocoon.
MALE REPRODUCTIVE ORGANS:
The male reproductive organs consist of testes, testes sac, seminal vesicle, spermiducal funnel, vasa differentia, prostate gland, accessory gland, genital papillae.
TESTES: The two pair of testis are located at different segments. One is located at the 10th segment and the other is located at the 11th segment. Production of sperms is the function of the Testes. These are contained in their respective testis sac. The testes produce spermatogonia or sperms.
TESTES SACS: The testes are contained in the testes sacs. These sacs are filled with fluid. Along with the testes, the spermiducal funnels are also located within the sac located at the 10th segment. Whereas, the sac located at the 11th segment encloses the testis, spermiducal funnel as well as the seminal vesicle of that segment. Hence the sac at the 11th segment is comparatively larger in size. The testes sacs are connected with the seminal vesicles through a duct. As the testes sacs develop from the body cavity as a small outgrowth, they become saccular on the sideways.
SEMINAL VESICLE: The spermatogonia are transferred to the testes sacs from the testes and then to the seminal vesicles where they develop into sperms. When the sperms are mature enough, they go back to the testes sacs and then into the spermiducal funnels. After spermatogonia undergo maturation division, sperm or spermatozoa are formed.
SPERMIDUCAL FUNNELS: Found just below the testes, the spermiductal funnels are ciliated, meaning that they have tiny hair-like projections. They are located at the 10th and 12th segments. From the seminal vesicle, the mature sperms travel back to the testes sac and move to vasa differentia through the spermiducal funnel.
VASA DIFFERENTIA: From the segments 12th to 17th, there are four tubes called vasa differentia. The duct of the prostate gland links with vasa differentia at the 17th segment. They are also ciliated internally.
PROSTATE GLAND: The prostate glands can extend from the 16th segment up to the 21st segment. They unite with the vasa differentia and opens at the genital fissure.
ACCESSORY GLAND: Located at the 17th and 19th segments, the accessory glands have small ducts which open through the papillae. These glands are small and round in shape.
GENITAL PAPILLAE: Located on the underside or the ventral side at the 17th and 19th segments are the pair of genital papillae which serves a copulatory function.
FEMALE REPRODUCTIVE ORGANS:
OVARIES: At the 13th segment there is a pair of the ovary which hangs between the 12th and 13th segment on a horizontal structure called septum. The eggs or the ova are contained in the ovaries in finger-like projections. In these projections, the ova which is mature is located at the end and the immature ova is located at the base.
OVIDUCAL FUNNEL: A pair of a funnel-like structure called the oviducal funnel is located at the 13th segment just below the ovary. They have a ciliated mouth. Both of the funnels open up into conical ducts called oviducts. At the 14th segment, two oviducts unite and then open through the female genital fissure.
SPERMATHECAE: Between the consecutive pairs of 5th-6th, 6th-7th, 7th-8th and the 8th-9th segments, four pairs of flask-shaped structures are located which are known as spermathecae. These structures are responsible for storing sperms.
THE REPRODUCTION PROCESS:
During the process of reproduction, the earthworms use both the female and male sex organs, so that there are more changes of fertilization since both sets of organs are being used by both the earthworms. Now, the course of reproduction is comprised of two processes, first being the process of mating known as copulation and the other is fertilization. During the mating process, one worm lines up against the other earthworm but in the opposite directions. As soon as this happens, each worm starts producing a huge amount of slimy liquid called mucus. The mucus forms a tubular structure around their bodies. since each worm consists of male sex organs, the ejaculation of sperms take place in both of the earthworms. The ejaculated sperms are the transferred through this slime tube and then deposited on the female pores of the other and after this, both the worms move away. This is where the process of copulation ends.
Now as the worms separate, a further process of reproduction takes place. Earthworms consist of a wide, band-like structure located at the front side, called the clitellum. The function of clitellum is to produce another tube of mucus. The clitellum will shape a slime tube around it, which will be filled with albuminus fluid. The eggs in the female pore will get attached to the slimy tube. The slime tube which now consists of the eggs passes over the spermathecae where the sperms of the other worms are stored. Now the eggs and sperms come into contact and fertilization takes place. The band of slime is now of no use and is shrugged off as it forms a cocoon around the eggs which are usually 30-40 in number. The development of these eggs takes place entirely in the cocoon. The cocoon contains so many eggs but only one of them develops. When this happens the other cells are used as a source of nutrients for the developing embryo. The cycle of reproduction can take place as often as every week or so. Earthworms can reproduce very frequently, their population can also double within a week.
Since earthworms consist of both the male and female sexual organs, the common misconception is that they can reproduce asexually, that is, with only one parent. An earthworm cannot produce a whole new organism on its own. The process of reproduction is done sexually. Since the day they are born, earthworms have to protect themselves from being eaten by many predators. Be it birds, rats, moles or any other small organism like hedgehogs, foxes, turtles, earthworms serve as food for many of them. They are also used as baits for fishes. They also serve as hosts for a number of parasites which often affect human beings, for example, nematodes, flatworms, mites, and flies. They lay their eggs in the burrows of earthworms. When these eggs hatch, the offspring attach themselves to the bodies of the earthworms and feed off them.