Phylum Annelida General Characteristics

Bizarre gills!!!

What are they? Round worm-like animals which are segmented. The name comes from the Latin “annellus” which means little ring. Appearance of the segments. A ring in French is call anneau.

Can range in size from less than 3mm to more than 3m! Mostly worm-like in appearance. The body segments are separated internally by walls of tissue called septa. Singular ~ septum. Most segments are similar, however some are specialized.

General features of a segment.

Annelids also have a true coelom like us, meaning that the entire body cavity is lined with mesoderm tissue. Platyhelminthes Nematoda Annelida This is of course a simplified diagram.

Digestive System A one-way digestive tract, so they have an anus. There are many specialized compartments. This is a general plan although it varies between species and classes. 1. Food enters the mouth. 2. Food enters the pharynx, a muscular tube. Can be protruded through the mouth. Carnivorous species have powerful jaws attached to lunge and catch prey!!!

Herbivorous worms also have jaws to bite off bits of algae. Nom nom nom!

Pharynx can act as a sticky surface or a pump for those annelids that are deposit feeders. (eat dirt/mud/sand) Can also be a pump to suck up blood and tissues in parasite species. 3. Food then enters the gut where it is moved by peristalsis. Muscular waves of the gut wall. 4. Food leaves via the anus.

Respiratory System Marine worms often breathe through gills called parapodia or parapodium (singular). Thin extensions of the body wall. Some marine worms can have large, fan-like gills. ~ see slide #2.

Circulatory System Possess a circulatory system! It is a closed circulatory system with contractile vessels. The system has two main blood vessels, a dorsal and ventral blood vessel. Located on top and underneath of the gut. Supply blood to the internal organs.

Dorsal blood vessel Ventral blood vessel

The earthworm has 5 contractile “hearts”. Pumping ring vessels. 5 “hearts”

Excretory System Solid wastes from digestion pass through the anus. Metabolic wastes such as ammonia are released via the metanephridia. More advanced versions of the protonephridia we saw in Platyhelminthes. Metanephridia: A tube that collects wastes from the body cavity and releases them outside. (A pair per segment) Earthworms also release wastes via their skin.

Metanephridia Metabolic wastes in. Metabolic wastes excreted.

Nervous System Many annelids are active animals with well developed nervous systems. They have a brain. The brain is at the front, (cephalization), and sits on top of the digestive tract. Associated with a nerve ring that goes around the gut with a pair of ganglia at the bottom. A large ventral nerve cord then runs the length of the body.

What does this remind you of? Each segment has nerves that branch off the ventral nerve cord and a pair of ganglia. What does this remind you of? Segmental nerves Brain Gut Nerve Ring Ventral Nerve Cord

Reproductive System Some annelids can reproduce asexually by budding, but not many. Marine species are often gonochoristic and have external fertilization. Earthworms and leeches are often hermaphroditic and have internal fertilization.