Earthworm: Reproduction

Earthworms reproduce sexually. Asexual reproduction is absent. However some power of regeneration is present. Though they are hermaphrodite, cross-fertilization takes place in them due to protandrous condition ( A condition  in which male gametes mature  earlier than female gametes in hermaphrodite organisms) . It is preceded by copulation and cocoon formation.

Copulation

It takes place during rainy seasons, generally at night or early morning and lasts for about one hour. During it, two earthworms come in contact of each other by their ventral surface facing anterior end towards opposite direction in such a way  that male genital pore of each earthworm lie against a pair of spermethecal pores of other. The male genital apertures are raised into papillae, which are inserted successively from behind to forward into the spermethecal pores of other earthworm and discharge the spermatic and prostatic fluid with sperms, which are stored in spermetheca.

After this exchange of sperms, the two earthworms separate.

Cocoon formation and fertilization

            Cocoon formation begins after copulation. During it, clitellum produces a type of viscous and gelatinous substance forming a broad membranous band around the 


clitellum. It hardens gradually on exposure to air and forms a tough but elastic tube which becomes the cocoon. As the earthworm wriggles back, the cocoon move forward.  on its way, concoon receives eggs from female genital pore and sperms of other earthworm from spermethecal pores. Albumen is also deposited inside concoon by the clitellum. Finally, cocoon comes out from the anterior end of body and its elastic ends close up to form a yellowish rounded cocoon.

Stages of cocoon formation

            Many concoons are formed in a season. The process of cocoon formation continues till sperms are present in spermetheca. Cocoon formation takes place in summer, during and after monsoon.

            Fertilization takes place after cocoons are deposited on land. Fertilization takes place inside cocoon. Thus, it is external.

Development :

            A cocoon may contains many fertilized eggs but only one develop, other degenerate and supply nutrients. The zygote undergoes successive mitosis division i.e. cleavage. Cleavage is holoblastic but unequal and development is direct. A hollow blastula is formed at first and it changes into gastrula. Young earthworm emerges out of cocoon in about 2-3 weeks. The new earthworm resembles the adult except for size and absence of clitellum.

The clitellum develops later on. Thus , development is direct.