Annelids

What is An Annelid? Belong to the phylum Annelida. Include earthworms, leeches, and marine worms. Annelids have segments separated by septa – internal walls between segments. The segments are very similar but they may be modified for specific reasons like a head or movement. They have a true coelom (body cavity) that is lined completely with mesoderm.

Form and Function in Annelids Annelids can range from filter feeders to predators. They have a pharynx that has 2 or more sharp jaws. They also have a crop where food can be stored. Lastly they have a gizzard that grind food into smaller pieces.

Annelids have a closed circulatory system – this means blood is moved through blood vessels completely.

Aquatic annelids have gills Aquatic annelids have gills. This is an organ specialized for gas exchange under water. Land dwelling annelids, like earthworms, do gas exchange through their skin. They will have a thin coating of mucus to keep their skin moist.

Annelids have a well-developed nervous system with a brain and several nerve cords.

Oligochaetes This class of annelids has the earthworms in it. They have few setae – little hairs that help them anchor into the dirt and move. Most live in soil or fresh water. Earthworms eat dirt and their poop is called castings. This is very important for fertilizing the soil and for aerating the soil.

Leeches This is class Hirudinea. Live in moist habitats in tropical countries. Leeches are external parasites that suck the blood and body fluids of their hosts. Leeches have suckers at both ends of their bodies to help them attach to their host. Some leeches release a substance that keeps the host from knowing they were bitten. Leeches can be used in medicine – they can reduce swelling after surgery. They also prevent blood from clotting so it can help with the reattachment of limbs.

Polychaetes Class Polychaeta – sandworms, bloodworms, marine worms. They have paired, paddle-like appendages with setae on the end. This is called parapodia.

Ecology of Annelids Earthworms will burrow through soil aerating it, and mixing it. This allows room for plant roots to grow and water to move so that many plants and bacteria can thrive. As mentioned before castings are a good fertilizer for soil. Many birds eat earthworms. There are other animals, like moles, skunks, toads, and snakes that eat earthworms.