Segmented Worms

• Annelida means “little rings” which is fitting because the bodies of the annelida are divided into similar rings or segments. 
• Although most of the annelids live in salt water, the earthworm is the most common from this phylum. 
• Like roundworms, these have an anus. 
• Along the ventral and lateral surfaces of the earthworm are setae (tiny bristles, four pairs to each segment) that can be retracted into the worm’s body. The worm moves by means of its setae and two muscular layers - a circular layer that makes the worm longer and thinner when contracted and longitudinal to make it shorter and thicker when they contract. Uses the setae as an anchor. 
• Nutrition: 
• Feeds on vegetation refuse and decayed animal matter in the soil. The worm sucks the soil in with its pharynx. It is moved to the gizzard where the food is ground into small pieces. Then moved to the intestine where enzymes break down the food. The nutrients are absorbed by blood circulating through the walls of the digestive tract as it moves through the intestine and out the anus.
• Circulation, Respiration, and Excretion
• The earthworm is the first animal we see with a circulatory system. In this case it is a closed system. It involves a series of vessels containing blood. It has two major vessels, the ventral and dorsal blood vessels. Near the mouth, there are five pairs of thickly muscled vessels called aortic arches that connect the dorsal and ventral blood vessels. These arches help regulate blood pressure.
• There are tiny vessels that bring oxygen absorbed through the epidermis to the rest of the tissues through its circulatory system. The earthworm has a protective cuticle that must be kept moist to allow to permit the passage of oxygen through it, but it can also suffocate if submerged in water, such as during a rain storm.
• To excrete waste, each segment of the worm except for the first three and the last, have nephridia, tubes that filter waste from the blood and expel them through small openings on the animal’s side called nephridiopores. 
• Nervous and Reproductive Systems
• The “brain” of the worm is actually a pair of ganglia, a mass of nervous tissue.
• A double ventral nerve cord extends from the large ganglion to the end of the worm. Along this cord are smaller ganglia, one for each segment of the earthworm.
• Earthworms are sensitive to light, touch, and certain chemicals. They do not have sensory organs, but they have sensory cells distributed all over their body.
• Earthworms prefer to reproduce sexually, although some species can regenerate.
• The earthworm is hemaphroditic. Their male and female sex organs are located in separate segments. 
• In sexual reproduction, like the flatworms, there needs to be cross-fertilization. 
• Some earthworms live to be several years old!