Phylum Annelida

• Annelida - also called segmented worms
• Annelida - came from the Latin word "annellus" which means ring and Greek word "aidos" which means form
• Annelida - any member of a phylum of invertebrate animals that are characterized by the possession of a body cavity (or coelom), movable bristles (or setae), and a body divided into segments by transverse rings, or annulations, from which they take their name
• Annelidao - found worldwide in all types of habitats, especially oceanic waters, fresh waters, and damp soils
• Most polychaetes live in the ocean, where they either float, burrow, wander on the bottom, or live in tubes they construct; their colors range from brilliant to dull, and some species can produce light.
• Annelida - the length of annelids varies from a fraction of an inch to more than six meters (about 20 feet) and the width may exceed 2.5 centimeters (about one inch) in the contracted state
• Annelids are tripoblastic.
• Gas exchange of annelids is through skin/gills/parapodia.
• Phylum Anmelida have 15000-22000 extant species.
• Two classes of Phylum Annelida - Polychaeta and Clitellata
• Polychaeta - the largest class of annelids, includes over 10,000 species of mostly marine and highly diverse segmented worms
• The word Polychaeta means "many bristled (setae)"
• Polychaetes are distinguished from other annelids by having pairs of appendages called parapodia attached to the outside of each segment. Parapodia are shaped like paddles, and they are used for both locomotion and respiration.
• Polychaets - most polychaetes do not have gills, and respiration occurs through the surface of the body, particularly through the parapodia
• The parapodia are filled with tiny blood vessels that can absorb oxygen through the surface of these appendages.
• The parapodia are also where the many bristles of polychaetes are located.
• Clitellata - are characterized by the presence of α specialized reproductive organ called the clitellum.
• Subclasses of Clitellata - Oligochaeta and Hirudinea
• Oligochaeta - the subclass Oligochaeta includes the well-known earthworms
• unlike polychaetes, oligochaetes do not have parapodia
• the word oligochaete means "few bristled"
• The first segment of the earthworm, the peristomium, contains the mouth.
• There is a small tongue-like lobe just above the mouth called the prostomium. Earthworms use the prostomium to see their environment, as earthworms have no eyes, ears, nose or hands.
• About one third of the way down the earthworm (from the head) is the clitellum. The clitellum is a swelling of the skin and can only be seen in earthworms that are ready to reproduce. It may be white, orange-red or reddish-brown in colour. Earthworms are ready to mate when their clitellum is orange.
• The very last segment is called the periproct and contains the anus.
• Except for the first and last segment, all the other segments have eight setae located around each segment. The setae look like small bristles sticking out of the earthworm’s skin. The setae can be retracted and are for moving through the soil. The bristle-like setae anchor the segments as they crawl.
• Anatomy of an Earthworm

  
• Earthworm species usually have four pairs of small bristles on each segment.
• There are over 3,000 species of oligochaetes, which makes up about one third of the annelid phylum. They range in size from a few centimeters up to three meters.
• Although the number of species of oligochaetes is much lower than the number of polychaete species, the actual population of oligochaetes is much greater. This is primarily due to the incredibly large population of these worms in some regions of the Earth's soil.
• Earthworms secrete a fluid, called 'coelomic fluid', to help them move underground and to keep them moist.
• The worms that make up the subclass Hirudinea are the species that we know as leeches.
• leeches are close relatives of oligochaetes
• Like oligochaetes, leeches lack parapodia, but, unlike oligochaetes, they also lack bristles.
• There are roughly 500 species of leeches. Many leeches are blood-sucking parasites, but there are also many species that are invertebrate predators.
• Leeches often eat their prey whole.
• Leeches have two structures called suckers, one located on the anterior, or front, region and one located on the posterior, or tail end, that are used for locomotion.
• For blood-sucking and consuming prey, leeches use a tubular feeding organ called a proboscis. One interesting feature of leeches is that they all have exactly 34 body segments.