Phylum Annelida

Welcome to the World of Annelids

Organisms in Phylum Annelida are commonly known as segmented worms.  There are 9,000 species in this phylum!  The 'rings' you see around annelids are their segments.  The segments are separated by septa.  Some annelids have segments with bristles, called setae, attached.   Each segment is similar to each other, though it may be specialized for certain functions.   Annelids have bilateral symmetry.  This means that if you were to draw a line from the anterior (front end) to the posterior (back end), it would make a mirror image on each side. 

These worms have a closed circulatory system.  In this case, this means that they have blood vessels to get oxygen around their bodies.  The dorsal, which means the top, blood vessel pumps blood, acting like a heart.

Their digestive system starts with the mouth.  Depending on the type of annelid, food is taken to the pharynx, which can contain sharp jaws.  After that, it travels along the esophagus and then to the crop.  The crop stores food until it can move to the gizzard where food is broken down more.  Finally, it goes to the intestines, where it stays, until it is pushed out of the anus.

Their nervous system is usually well-developed.  They have a brain and a series of nerve chords called ganglia.  Some of their sensory organs include two or more eyes, sensory tentacles and chemical receptors.

Annelids typically reproduce sexually.  Some annelids have different sex organs, while others are hermaphrodites and have both kinds of sex organs.  Even if they have both, they still exchange sperm to fertilize each other's eggs which contributes to genetic diversity.  Eggs are stored in a specialized group of segments call a clitellum.  You can clearly see it on an earthworm, as the thick band located close to the anterior end.

Classes of Annelida: Polychaeta, Oligochaeta, and Hirudina



Class Polychaeta

Class Polychaeta holds the most species of annelids at around 8,000 species!  These are marine worms that have bristle like appendages, called parapodia, with setae at the ends.  They can be found from the surface to the depths of the ocean. 

Class Hirudina

Class Hirudina contain the parasitical leeches.  Leeches use powerful suckers to suck blood and other body fluids from their hosts.  Some have a proboscis, which is a muscular extension, while others use sharp jaws to cut into their host.  Sometimes leeches use a numbing substance on the wound so that their hosts don't realize they are there.



Class Oligochaeta

Class Oligochaeta are made up of mainly terrestrial (land) annelids.  They can be found on land in aquatic areas, or on land in the soil.  Earthworms are a part of Class Oligochaeta.

Leeches Make a Comeback

Surprisingly, one interesting part of Phylum Annelida are the leeches.  3,000 years ago leeches started being used as medical treatments.  The doctors would use leeches to suck the "bad" blood away.  They didn't realize that this would also take the "good" blood away as well.  This practice of bloodletting stopped in the late 19th century.  However, leeches have made a comeback in the medicine world.  When leeches attach themselves to a host they secrete a variety of biological compounds.  These compounds help to prevent blood clotting, reduce swelling, and more.  Now, they are being used as a new remedy to help combat against cardiovascular problems, cancer, metastasis, and infectious diseases.  They are also used in some plastic and microsurgeries to protect against venous congestion.  Though, there is still a debate on the safety and effectiveness of leeches, it is interesting to see how the use of leeches have evolved over the years.