Phylum Annelida
General Info:
 Roughly 12400 species of annelids
 defined by a segmented worm-like body that is soft
 Most live in the sea or under the ground
 Range in size from half a millimeter long to 3 meters long.
 Bilateral
 Coelomates
Form and Function:
Digestive System:
 Have a complete digestive gut from mouth to anus
 Just inside the mouth is a pharynx that can extend through the mouth to bring in food
o Many species have two or more sharp jaws attached to it
 Carnivores grab onto prey
 Herbivores tear off algae
 Detritus feeders have a mucus-covered pharynx that food particles stick to
 Some annelids have specialized structures that allow
them to filter feed.
 From the pharynx food enters an esophagus
 Food then enters a crop (food storage)and a gizzard
(grinds food into small pieces)
 The bits of food then enter the intestine where the food
is digested
 Any undigested food is eliminated through the anus.
 Earthworms eat just about anything that is in the soil
Respiratory System:
 Aquatic species often breath through gills
 Terrestrial annelids can perform gas exchange through their skin. The skin must stay
moist in order for this exchange to occur.
o Terrestrial annelids secrete a cuticle that protects from drying out.
Circulatory System:
 Closed circulatory system of two main blood vessels
that run the length of the body
 Blood moves towards the head of the worm in the top
(dorsal) vessel and towards the tail of the worm in the
bottom (ventral) vessel.
 There are rings of blood vessels that connect the two
main vessels that are similar to hearts. They contract
rhythmically to pump blood throughout the body.
 In some annelids that blood is pushed through the body by muscle contractions during
movement.
Excretory System:
 Metabolic wastes are excreted through specialized structures called nephridia.
 There is a pair of nephridia in each body segment
Nervous System:
 Well-developed nervous system
 The brain sits above the gut in the head
region
 Two large nerves pass around the gut to a
pair of ganglia located beneath the gut.
 From these ganglia, a ventral nerve cord
runs the entire length of the body.
 There are smaller ganglia in each section
where smaller nerves attach to the main
cord.
 Other annelids have further specialized sensory structures such as sensory tentacles,
statocysts (balance/orientation), chemical receptors, or simple eyes that can detect light.
Locomotion:
 Two major groups of muscles in their body walls
o Longitudinal muscles run from front to rear. When they contract, the worm gets
shorter
o Circular muscles run in circles around the body of the worm. When these muscles
contract, the worm gets skinnier
Reproduction:
 Some annelids can reproduce asexually by budding but most reproduce sexually


Some species have separate sexes and
fertilization occurs externally. The
female releases an egg into the open and
the male releases sperm.
Other annelids are hermaphrodites that
undergo internal fertilization.
o Two worms pair up and there is a
two-way exchange of sperm
o The worm has a segment called a
clitellum which secretes a mucus
ring in which the eggs are
fertilized with sperm
Class Polychaeta:
 Polychaeta means many bristles
 Characterized by two paddle-like appendages on each segment that have bristles on the
ends.
Class Oligochaeta:
 Earthworms and relatives.
 Smaller and fewer bristles on the ventral side provide anchorage for the worm
 Most live in soil and freshwater
Class Hirudinea:
 Contains leeches.
 Mostly tropical
 External parasites that feed on the blood of their hosts
 Some leech species are carnivorous and they feed on smaller soft-bodied animals.
o They either suck the body parts from the animal or eat it whole.