Model Answer
0 min readIntroduction
Pheretima, commonly known as the earthworm, is a vital invertebrate belonging to the phylum Annelida. These terrestrial organisms are renowned for their significant role in soil aeration and nutrient cycling, often referred to as "farmer's friends." Despite being hermaphroditic, meaning each individual possesses both male and female reproductive organs, Pheretima primarily undergoes cross-fertilization to prevent self-fertilization, largely due to a phenomenon called protandry where male gonads mature earlier than female gonads. Their life cycle is characterized by direct development, without any larval stages, culminating in the emergence of miniature adults from protective cocoons.
Life Cycle of Pheretima
The life cycle of Pheretima involves several distinct stages, from sexual reproduction to the development of new individuals.1. Copulation
Copulation is the initial step in the reproductive cycle of Pheretima. Earthworms are nocturnal and breed during the rainy season. During this process, two mature earthworms align themselves ventrally, with their anterior ends pointing in opposite directions. The male genital openings of one worm are positioned opposite the spermathecal pores of the other. This allows for a mutual exchange of spermatozoa and prostatic fluid. Special glandular papillae surrounding the male genital pores facilitate the insertion into the spermathecal pores, ensuring the transfer of sperm. The transferred sperm is stored in the spermathecae of each worm.
Diagram: Copulation in Earthworms
[Imagine a diagram showing two earthworms aligned in opposite directions, ventral surfaces touching. Arrows indicate mutual sperm exchange from male genital pores (segment 18) to spermathecal pores (segments 5/6, 6/7, 7/8, 8/9).]
2. Cocoon Formation
After copulation, the clitellum, a prominent glandular band typically located on segments 14-16 in a mature worm, becomes highly active. The glandular cells of the clitellum secrete a viscid, gelatinous substance that hardens upon exposure to air, forming a protective structure known as a cocoon or ootheca. This cocoon acts as a protective capsule for the developing embryos.
As the worm slowly withdraws backward, the secreted cocoon slides forward over its body. During this passage, the cocoon collects the worm's own eggs from the female genital aperture (located on segment 14) and the stored sperm received from the partner, which is released from the spermathecae (segments 5-9) into the cocoon. This ensures that both ova and spermatozoa are enclosed within the cocoon.
Diagram: Cocoon Formation
[Imagine a diagram showing a worm with a secreted cocoon sliding off its anterior end. The cocoon is depicted as collecting eggs from the female genital pore and sperm from spermathecal pores.]
3. Fertilization and Embryonic Development
Fertilization in Pheretima is external and occurs within the cocoon. Once the cocoon slips off the anterior end of the worm, its ends seal, creating a closed environment. Inside this cocoon, the eggs are fertilized by the collected sperm. The clitellum also secretes albumin, which serves as nourishment for the developing embryos.
The zygote undergoes direct development, meaning there are no larval stages. The embryonic development involves typical stages of cleavage, blastulation, and gastrulation. The embryos feed on the albumin within the cocoon.
4. Hatching of Juveniles
After an incubation period, which can range from a few weeks to several months depending on environmental conditions like temperature and moisture, several juvenile worms hatch from the cocoon. These young worms are miniature versions of the adult, but they lack a clitellum. They gradually grow and mature, developing their clitellum once they reach sexual maturity, thus completing the life cycle.
Summary of Life Cycle Stages
| Stage | Description | Key Events |
|---|---|---|
| Copulation | Two earthworms align to exchange sperm. | Mutual sperm exchange, sperm stored in spermathecae. |
| Cocoon Formation | Clitellum secretes a protective capsule. | Cocoon formed, collects eggs and stored sperm. |
| Fertilization & Development | Eggs fertilized within cocoon, embryo development. | External fertilization, direct development, albumin nourishment. |
| Hatching | Juvenile worms emerge from cocoon. | Miniature adults hatch, grow and mature. |
Conclusion
The life cycle of Pheretima exemplifies a fascinating reproductive strategy among invertebrates, characterized by its hermaphroditic nature yet reliance on cross-fertilization. The intricate process of copulation, followed by the ingenious mechanism of cocoon formation, ensures the protection and nourishment of developing embryos. The direct development, bypassing larval stages, highlights the efficiency of their reproductive biology. Understanding these stages is crucial not only for zoological study but also for appreciating the ecological role of earthworms in maintaining soil health and fertility, making them invaluable members of terrestrial ecosystems.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.