Phylum Annelida includes approximately 9,000 species of freshwater, marine, and terrestrial worms. The most distinctive feature of this group is the division of their body into segments, or metameres. The metameres are visible rings on the outside and are separated by partitions (septa) on the inside.
        Annelids have a segmented coelem, a tubular gut, and a closed circulatory system that transports oxygen and food throughout the body. The excretory system consists of paired nephridia, tubular excretory structures, typically located in each segment except the head. They also have a centralized nervous system and a multiple sensory cells, including touch cells, taste receptors, light-sensitive cells, and cells that detect moisture. Some also have well-developed eyes and sensory antennae.
 

Class Oligochaeta: The Earthworms
 

        The body of an earthworm is divided into regular segments, of which most are identical. Each identical segment contains two nephridia, three pairs of nerves branching off from the central nerve cord running along the ventral surface, a portion of the digestive tract, and a left and right coelomic cavity; each segment also bears four pairs of bristles, or setae.

    Digestion in Earthworms

      The digestive tract of the earthworm is a long, straight tube. The mouth leads into a strong, muscular pharynx, which acts like a suction cup, helping the mouth to draw in organic matter. The narrow section of the digestive tract posterior to the pharynx, the esophagus, leads to the crop, where food is stored. In the gizzard, which has thick, muscular walls lined with protective cuticle, the food is ground up with the help of the ever-present soil particles. The rest of the digestuve tract is made up of a long intestine.

    Circulation in Earthworms

        The circulatory system of the earthworm is composed of longitudinal vessels running the entire length of the worm, one dorsal and several ventral. Connecting the dorsal and ventral vessels, and so completing the circuit are five pairs of hearts, muscular pumping areas in the blood vessels. Both the hearts and the dorsal vessel have valves that prevent backflow. Note that annelids have a closed circulatory system in which the blood flows entirely through vessels.

    Respiration in Earthworms

      Respiration in earthworms takes place by simple diffusion through the body surface. Oxygen diffuses inward to the network of capillaries just underlying the body surface and is consumed by body cells as the blood circulates. Carbon dioxide, which is picked up by the blood, diffuses out into the air through the body surface.

    Excretion in Earthworms

      The excretory system of the earthworm consists of one pair of nephridia for each segment. Each nephridium consists of a long, convolted tubule that begins with a ciliated funnel opening into the coelomic cavity of the anteriorly adjacent segment. Coelomic fluid is carried into the funnel by the beating of the cilia and is excreted through an outer pore.

    The Nervous System of Earthworms

      Earthworms have a variety of sensory cells. Touch cells, or mechanoreceptors, contain tactile hairs, which, when stimulated, trigger a nerve impulse. The earthworm also has light-sensitive cells. Such cells are more abundant in its anterior and posterior segment, the parts of its body most likely to be outside of the burrow. One of the earthworm's most sensitive cells are those that detect moisture. These cells are located on the first few segments. The animal also appears to have taste cells. In the laboratory, worms can be shown to select celery in preference to cabbage leaves.
        Each segment of the worm is supplied by nerves that receive impulses from the sensory cells and by nerves that cause muscles to contract. The cell bodies for these nerves are grouped together in clusters (ganglia).

Class Polychaeta
 

     The polychaetes, which are almost all marine, differ from earthworms and other oligochaetes in a number of ways. First, they usually have a variety of appendages, including tentacles, antennae, and specialized mouth parts. Also, each segment contains two fleshy extensions, parapodia, which function in locomotion and gas exchange. In polychaetes and other segmented animals, there is a tendency for the division of labor between segments to lead to tagmosis-the formation of groups segments into body regions with functional differences.
        Polychaetes have diverse life styles. Some are motile predators, others feed on materials suspended in the water or deposited in bottom sediments. Usually the sexes are separate, fertilization is external, and there is a free-swimming trochophore larva.
 

Class Hirudinea
 

     Hirudineans are the leeches, which have flattened, often tapered, bodies with a sucker at each end. These animals move along with a loping movement or swim with undulating motions of the body. Also, leeches are hermaphrodites.
     Bloodsucking leeches attach themselves to their hosts by their posterior sucker, and then, using their anterior sucker, either slit the host's skin with their sharp jaws or digest an opening through the skin by means of enzymes. Then they secrete chemicals that prevent the formation of blood clots into the host's blood. One of these chemicals, hirudin, is the most powerful anticoagulant (substance that prevents blood clots) known. Another substance in leech saliva has been demonstrated to inhibit the spread of malignant cells from lung cancers.
 

Thanks to the following sites for the pictures: http://ag.arizona.edu/tree/eukaryotes/animals/annelida/annelida.html http://www.vims.edu/~mes/leech/hirudinea.html  http://utweb.utampa.edu/faculty/kbeach/bio203lab/animallab2/sld016.htm