The phylum Annelida is made up of segmented worms, numbering about 15,000 species. Body segmentation, a hallmark of

annelids, was a major step in the evolution of animals. Annelids are protosomes, meaning they have a coelom made from cell

masses. This coelom is divided into a series of repeated parts. This repetition is called metamerism, and each segment is called a

metamere. There are a cluster of nerve cells and excretory organs in each metamere, but the ventral nerve cords, a dorsal and

ventral blood vessel, and the digestive tract pass through the walls of segmentation and are therefore unsegmented. These walls, or

septum, are thin sheets of mesodermic tissue, isolating the coelom. Except for the head and tail region, each with an opening of the

digestive tract, making it a complete tract, each segment in an annelid is ringlike and very similar.


Segmentation allows for flexibility and mobility because annelids can bend at segmented parts. Therefore, because a segmented

body is advantageous, it evolved twice, with the protosomes, as shown by phyla Annelida and Arthropoda, and again in the

deuterosomes, as shown by phylum Chordata. Other hallmarks of the annelids are soft bodies that are round in cross section,

repetition of organs in the segemented parts, and a body that is much longer than it is wide. There are three main classes in the

phylum Annelida.


Class Polychaeta –Polychaetes


Class Polychaeta consists of mostly marine annelids. Polychaetes are the largest group of annelids, and have hard bristles that

allow the worms to wriggle and move. However, only one type of polychaetes actually move. The Errantia are active and mobile,

while the Sedentaria are immobile, spending their entire lives burrowed in sediment. The appendages also are used in exchange of

oxygen and wastes by increasing surface area. Many polychaetes, especially the group Sedentaria, live in tubes that they make

from sticky proteins secreted near the mouth. Feathery appendages extending from the tubes trap food in the water. An example of

a tube-building polychaete is a Christmas tree worm. Polychaetes usually have at least one set of eyes, one set of sensory organs,

and are of one sex.


Class Oligochaeta Oligochaetes


Class Oligochaeta consist of earthworms and other worms that live in terrestrial or freshwater environments. As shown by the class

name (meaning few setae, or appendages), these worms lack the bristles that allow movement and increase surface area. There are

some that are marine, but most (94%) live out of sea water. Also, all oligochaetes are hermaphroditic, neither male nor female. One

oligochaete, the earthworm, extracts nutrients from the soil as it burrows, excreted from the anus. Because the nutrients that

earthworms dig up are necessary for fertile soil, these earthworms are an intricate part of farms, actually tilling the soil.


Class Hirudinea Leeches


The final major class in phylum Annelida, class Hirudinea, consists of the leeches. Leeches are known for sucking the blood of

humans, but most are free-living and eat small invertebrates, feeding on their blood. Most live in freshwater environments, although a

few can live in terrestrial environments. Like earthworms, leeches are hermaphroditic and lack appendages. To successfully drink the

blood, they have a small sucker in the anterior end and a larger one in the posterior end.












dorsal blood vessel


aortic arches


ventral blood vessel


ventral nerve cord






cephalic ganglia