Reproduction in Annelida

In Polychaeta sexes are separate. The gonads are patches of coelomic epithelium and are repeated in most of the segments. The gonads become conspicuous during the breeding season and they proliferate a very large number of germ cells which detach and fill coelom where they undergo maturation in the coelomic fluid.

When ripe the germ cells pass to the exterior either through segmental organs or by rupture of the body wall. Fertilisation occurs in sea water. In many forms a phenomenon of swarming occurs, the crawling or burrowing worms rise to the surface to discharge their sex cells, then they sink to the bottom.

Swarming habit is an adaptation for securing fertilisation of the greatest possible number of eggs. Swarming is usually during definite periods and often coincides with lunar periods. Discharge of gametes is nearly always followed by death of the sexual individuals. Fertilised egg gives rise to a trochosphere larva.

In syllids gonads are usually confined to the posterior part of the body which is detached as a free-swimming zooid which develops a head but no jaws or pharynx, it lives for a time to produce gametes. Many annelids have the ability to regenerate lost parts, this is accompanied by a capacity to reproduce asexually.

Some forms reproduce asexually by budding, but in Autolytus there is a proliferating region at the end from which a chain of sexual zooids is budded off which detach one by one. Syllis forms many branches by budding, some of which form a head, develop sex organs, notopodia are formed to reconstruct the parapodia, these sexual forms may remain attached to the parent for long or they may separate from the colony.

In Oligochaeta certain features of reproductive organs are salient characters, they are almost all hermaphrodite.

The sex cells are discharged into the coelom or into seminal vesicles which are special parts of the coelom separated from the rest, they are large coelomic sacs varying in number in different genera; often a pair of seminal vesicles may coalesce to form a median sperm reservoir into which ciliated funnels of vasa deferentia open.

Testes may be several but ovaries are never more than two. Spermathecae are usually present to receive spermatozoa of another worm during copulation.

The clitellum is a glandular development of the epidermis for formation of cocoons and albumen for nourishment of the embryo.

The clitellum may be permanent, as in earthworms, or it may develop only during the breeding season. Some Oligochaeta possess special copulatory setae. In some Oligochaeta asexual reproduction occurs, e.g., Nais and Chaetogaster multiply by proliferation of segments at the posterior end forming a chain of zooids which eventually separate and acquire sex cells.

Hirudinea are hermaphrodite with several pairs of testes but only two ovaries, gonads are completely shut off in closed coelomic vesicles, but they are continuous with their ducts in distinction from other Annelida. The spermatozoa unite in bundles to form permatophores.

Generally copulation occurs, though in some hypodermic impregnation takes place. The clitellum appears during breeding season, and eggs are laid in cocoons formed by clitellar glands.

Archiannelida are generally hermaphrodite, the ovaries occurring in anterior segments and testes behind them, so that gonads are restricted to a few segments. In Polygordius sexes are separate, the ovaries or testes develop in a few posterior segments, there are no ducts.