The annelids are the segmented worms, which include the common earthworm. Segmentation facilitates the formation of larger organisms and the motility of those organisms. Annelids have a completely closed circulatory system, which allows higher blood pressures and the consequent control over blood flow. They lack specialized respiratory organs. Their digestive system is more developed than that of other phyla we have discussed, containing two specialized areas: The crop stores food; the gizzard grinds it. Annelids are usually divided into three classes: polychaetes, oligochaetes, and hirudinea.
Polychaetes (many bristles) include 10,000 species, mostly of marine worms.They are usually 5 to 10 cm long, and many are brightly colored. They are distinguished from the other annelids by the presence of a pair of appendages on each segment. They are commonly found burrowed in the sediments of estuaries. They may achieve densities of thousands per square meter in mud flats. Others move about, and these are usually predatory.
Oligochaetes (few bristles) are the class that includes the common earthworm Lumbri-cus terrestris. Most are scavengers, feeding on decayed plant and animal matter. Earthworms are hermaphroditic but must copulate with another individual. Eggs and sperm are then deposited in a mucus and chitin pouch, or cocoon. Earthworms play an important role in the ecology of the soil. Their burrows improve soil drainage and aeration. They mix the soil by ingesting it at depth and depositing it at the surface, and they bring organic matter from the surface down into the burrows, where the nutrients released by their decay can become available to plant roots. Freshwater oligochaetes are smaller but more mobile than their terrestrial cousins. They can be an important source of food for fish. One is Tubifex, a small reddish worm that forms tubes in the sediment in which it lives head down, with tails waving in the water. Tubifex can form carpets on the bottoms of heavily polluted streams.
The class hirudinea are the leeches, most of which live by sucking blood from other organisms. They have specialized sucker organs to attach themselves to prey. Some aquatic leeches prey on fish, cattle, horses, and humans. Some terrestrial leeches prey on insect larvae, earthworms, and slugs, or may climb bushes and trees to reach birds.
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