The annelids possess a segmented body pattern and form an extremely diverse group of worms. The habitat of annelids may be terrestrial or aquatic---both marine and freshwater. The annelids include marine polychaete worms, freshwater leeches and earthworms. Most species have a simple life cycle, with the exception of the marine polychaetes.
With the exception of the marine polychaetes, annelids are hermaphrodites: Each individual has male and female reproductive organs. Still, an annelid cannot reproduce without a contribution from a mate. Polychaete worms are either male or female.
Earthworms, perhaps the most familiar annelids, mate before laying eggs. Two worms bind themselves to each other while each worm passes a sperm packet to the other. After mating, the broad, saddle-like band on the worm (called the clitellum) secretes a mucus sheath that begins to move toward the head of the worm. As it moves forward, the worm secretes sperm and eggs into the sheath, which eventually forms an egg cocoon. Terrestrial annelids lay their eggs in the soil, whereas aquatic annelids deposit or attach their egg cocoon to plants or to the soil substrate. Marine polychaetes transform into a reproductive stage called an epitoke before mating. The polychaete's male and female epitokes release sperm and eggs into the water.
Marine polychaetes have a free-living larval stage, called a "trochophore." The trochophore eventually transforms into the adult form.
Newly hatched or metamorphosed annelids settle into adult habitats. Most adult annelids live in the soil. Marine polychaetes live in the soil substrate of their aquatic habitat. Some marine polychaetes create tubes in the mud, and these somewhat rigid tubes provide protection. Other parasitic annelids are free-living.
Most adult annelids ingest soil, digest organic nutrients and excrete the inorganic leftovers---sand particles, for example. Some parasitic species such as leeches, however, feed on other organisms. A few species even prey on other invertebrates.
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