Phylum Annelida

Phylum Annelida

• What is a segmented worm?

• This phylum includes leeches, bristleworms, and earthworms.

• They have bilateral symmetry, have a coelom, and have 2 body openings.

Phylum Annelida

• The basic body plan of segmented worms is a tube within a tube.

• The internal tube is the digestive tract. • Food is taken in through the mouth, and waste is

released through the anus.

Phylum Annelida

• Most segmented worms have tiny bristles called setae on each segment.

• These help the worm move by providing a way to anchor their bodies in the soil so each segment can move the animal along.

Phylum Annelida

• Segmented worms can be found in most environments, except in the frozen soil of the polar regions and the dry sand and soil of the deserts.

• There are about 15,000 species of segmented worms.

A. Segmented supports diversified functions

• The most distinguishing characteristic of segmented worms is their cylinder shaped bodies that are divided into ring segments.

• This segment continues internally as each segment is separated from others by a body partition (or divider).

• This is an important adaptation for movement because each segment has its own muscles, allowing the body to shorten and lengthen.

Diversified Functions

• Segmentation also allows for specialized body tissues.

• Groups of segments may be adapted for a specific function.

• Certain segments are adapted to things such as sensing and reproductive system.

B. Nervous System

• Segmented worms have simple nervous systems. • Some sensory organs are sensitive to light, and

some species have eyes with lenses and retinas. • Some species have a brain located in an anterior

segment.• Nerve cords connect the brain to nerve centers

called ganglia.

C. Circulation and Respiration

• Segmented worms have a closed circulatory system.

• These worms must also live in water or in wet areas on land because they also exchange gases directly through their skin.

D. Digestion and Excretion

• Segmented worms have a complete internal digestive tract that runs the length of the body.

• Food and soil taken in by the mouth eventually pass to the gizzard.

• In this structure, muscles and hard particles help grind soil and food before they pass into the intestine.

• Undigested material and solid wastes pass out the worm’s body through the anus.

D. Digestion and Excretion

• Segmented worms have 2 nephridia in almost every segment that collect waste and help transport it out of the body.

E. Reproduction in Segmented Worms

• Earthworms and leeches are hermaphrodites. • During mating, two worms exchange sperm. • Each worm forms a capsule for the eggs and

sperm. • The eggs are fertilized in the capsule, then it slips

off the worm and is left behind in the soil.• In 2-3 weeks, young worms emerge from the

eggs.

E. Reproduction in Segmented Worms

• Bristleworms and their relatives have separate sexes and reproduce sexually.

• Usually eggs and sperm are released into the seawater, where fertilization takes place.

Diversity of Segmented Worms

• Phylum Annelida includes 3 classes:

• Class Oligochaeta: earthworms• Class Polychaeta: bristleworms• Class Hirudinea: leeches

A. Earthworms

• Earthworms are the most well-known annelids because they can be easily seen.

• Earthworms do not have a distinct head.• Earthworms eat their way through soil. • As they eat, they create spaces for air and water

to pass through soil. • As soil passes through their digestive tracts,

nutrients are extracted and undigested materials pass out of the worms.

• Castings, which is the waste of a worm, help fertilize soil.

B. Bristleworms

• Bristleworms and their relatives include fanworms, lug worms, plumed worms, and sea mice.

• This class is mostly marine worms. • Most body segments have many setae. • The name Polychaete means “many bristles”. • Most body segments of a polychaete also have a

pair of appendages called parapodia, which can be used for swimming or crawling over corals and the bottom of the sea.

• Parapodia also function in gas exchange.• A polychaete has a head with well-developed

sense organs, including eyes.

C. Leeches• Leeches are segmented worms with flat bodies and usually

no setae. • Most leeches live in freshwater streams or rivers.• Many leeches are parasites that suck blood or other body

fluids from their hosts. • Hosts include ducks, turtles, fishes, and humans. • Front and rear suckers enable leeches to attach themselves

to their host. • The bite of a leech is not painful. • This is because the saliva of the leech contains chemicals

that act as an anesthetic.• Other chemicals prevent the blood from clotting.• A leech can ingest 2-5 times its weight in one meal. • Once fed, a leech will drop off its host and may not eat

again for months.