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.