THE reproduction é sexual, being the species often (oligoquetos and hirudines) hermaphrodite and with direct development.
There are, however, forms with separate sexes and indirect development, through a trochaphous larva and some cases of asexual reproduction (polychaetes). Fertilization is always external.
Sexual reproduction can be illustrated with the process in hermaphrodite worms:
In the ventral anterior zone there are 3 or 4 pairs of openings for designated muscle seminal receptacles. These pouches store sperm received from a partner during copulation. In the clitoral region there is a pair of female gonopores, internally linked to funnel-shaped oviducts. These capture from the celoma the eggs produced by the ovaries.
The male apparatus consists of two pairs of testicles, associated with 2 to 4 pairs of seminal vesicles through a pair of seminiferous tubes or spermiducts. There is even a pair of prostate glands. This system opens abroad in the segment following female openings in male gonoporos.
The sperm produced by the testicles migrate to the seminal vesicles, where they mature and await mating. During copulation, they pass through the prostate glands, mingling with the nourishing liquids they produce, and are eliminated by the male gonoporos.
To facilitate the alignment of animals during copulation, there are suction cup-like structures called genital papillae, associated with male sexual openings. The clit also plays a role at this level, with its mucous secretions holding the animals together.
The copulation takes place between two animals ventrally united and oriented in opposite directions. In this way the male gonopores are aligned with the openings of the seminal receptacles. After reciprocal exchange of sperm, the worms separate.
Each animal will then produce a mucous ring from the clitellus. By contractions of the body, this ring is pushed forward, passing through the female sexual openings, which release the eggs, and the seminal receptacle openings, which release the sperm.
Then the ring will be released from the front end of the animal and form a protective cocoon where external fertilization takes place. This cocoon is about 1 cm long and looks like a small white ring. From it will appear small worms without larval stages.
Polychaete worms are the largest and most diverse class of annelids, but although abundant and often brown in color, they are rarely seen as they often live buried and escape quickly when disturbed. There are two main ones, the free-living ones such as the nereis, active-living, parapod-eating carnivores, and the sedentary tubercars living in tunnels or tubes they segregate, filtering their food with specialized parapods.
The wormlike make up the other large group of annelids and include earthworms and leeches. The name derives from the presence of the clitellum, a glandular band, that secretes a cocoon where fertilized eggs develop. This structure is obviously an adaptation to life on land where aquatic larvae would not survive. Unlike polychaetes, wormholes do not have parapods or tentacles on their heads.
Annelids can be terrestrial (wet soil) such as earthworms, marine like polychaete worms, which can be found near beaches or in deep water, or freshwater such as leeches. They may also be free-living, other aquatic animals or ect and endoparasite eaters. Some of the smallest representatives of this phylum are less than 1 mm long, but the giant worms in Brazil and Australia are 2 m long and 2.5 cm in diameter. Equally gigantic are some 3 m long polychaete worms and some 20 cm leeches.
The presence of some types of red earthworms in muddy areas is a positive indicator of the presence of pollution by organic debris.
Of the three major groups of protostomic animals, annelids are the smallest phylum, with only about 15,000 species known. However, they are the most advanced and most successful group of worms, having suffered their highest adaptive radiation at sea, although they are abundant on land and in freshwater.