Phylum Annelida: Segmented Worms

Section 27.2 Summary – pages Segmented worms are classified in the phylum Annelida. They include leeches and bristleworms as well as earthworms. Segmented Worms (Earthworms, Bristleworms, and Leeches) Segmented Worms (Earthworms, Bristleworms, and Leeches) Segmented worms are bilaterally symetrical and have a coelom and two body openings.

Section 27.2 Summary – pages The basic body plan of segmented worms is a tube within a tube. The internal tube, suspended within the coelom, is the digestive tract. Body Structure Food is taken in by the mouth and wastes are released through the anus.

Section 27.2 Summary – pages Most segmented worms have tiny bristles called setae on each segment. The setae help segmented worms move. They anchor their bodies in the soil so each segment can move the animal along. Setae Body Structure

Section 27.2 Summary – pages The most distinguishing characteristic of segmented worms is their cylindrical bodies that are divided into ringed segments. In most species, this segmentation continues internally as each segment is separated from the others by a body partition. Body Structure Segments

Section 27.2 Summary – pages Each segment has its own muscles, allowing shortening and lengthening of the body. Segmentation also allows for specialization of body tissues. Certain segments have modifications for functions such as sensing and reproduction. Body Structure

Section 27.2 Summary – pages Segmented worms have simple nervous systems in which organs in anterior segments have become modified for sensing the environment. Nervous system Some sensory organs are sensitive to light, and eyes with lenses and retinas have evolved in certain species.

Section 27.2 Summary – pages In some species there is a brain located in an anterior segment. Nerve cords connect the brain to nerve centers called ganglia, located in each segment. Setae Nerve Brain Nervous system Segments

Section 27.2 Summary – pages Segmented worms have a closed circulatory system. Circulation and Respiration Blood carrying oxygen to and carbon dioxide from body cells flow through vessels to reach all parts of the body. Segmented worms must live in water or in wet areas on land because they exchange gases directly through their moist skin.

Section 27.2 Summary – pages Segmented worms have a complete internal digestive tract that runs the length of the body. Digestion and Excretion Food and soil taken in by the mouth eventually pass to the gizzard.

Section 27.2 Summary – pages In the gizzard, a muscular sac and hard particles help grind soil and food before they pass into the intestine. Mouth Crop Gizzard Digestion and Excretion Circulatory System

Section 27.2 Summary – pages Undigested material and solid wastes pass out the worm’s body through the anus. Segmented worms have two nephridia in almost every segment. These collect waste and transport it through the coelom and out of the body. Nephridia Digestion and Excretion

Section 27.2 Summary – pages Earthworms and leeches are hermaphrodites, producing both eggs and sperm. Reproduction During mating, two worms exchange sperm. Each worm forms a capsule for the eggs and sperm.

Section 27.2 Summary – pages The eggs are fertilized in the capsule, then the capsule slips off the worm and is left behind in the soil. In two to three weeks, young worms emerge from the eggs. Reproduction

Section 27.2 Summary – pages Bristleworms and their relatives have separate sexes and reproduce sexually. Reproduction

Section 27.2 Summary – pages Usually eggs and sperm are released into the seawater, where fertilization takes place. Bristleworm larvae hatch in the sea and become part of the plankton. Once segment development begins, the worm settles to the bottom. Reproduction

Section 27.2 Summary – pages Annelida includes three classes: class Oligochaeta, earthworms class Polychaeta, bristleworms and their relatives class Hirudinea, leeches Diversity of Segmented Worms

Section 27.2 Summary – pages Earthworms are the most well-known annelids because they can be seen easily by most people. Oligochaeta: Earthworms As an earthworm burrows through soil, it loosens, aerates, and fertilizes the soil.

Section 27.2 Summary – pages Polychaeta: Bristleworms Most body segments of a polychaete have many setae, hence the name. Polychaete means “many bristles”. A polychaete has a head with well-developed sense organs, including eyes.

Section 27.2 Summary – pages Most body segments also have parapodia, which can be used for swimming or crawling over corals and the bottom of the sea. Polychaeta: Bristleworms Parapodia also function in gas exchange.

Section 27.2 Summary – pages Leeches are segmented worms with flattened bodies and usually no setae. Hirudinea: Leeches Many are parasites that suck blood or other body fluids from the bodies of their hosts (ducks, turtles, fishes, and humans).

Section 27.2 Summary – pages The saliva of the leech contains chemicals that act as an anesthetic. Other chemicals prevent the blood from clotting. A leech can ingest two to five times its own weight in one meal. Hirudinea: Leeches Front and rear suckers enable leeches to attach themselves to their hosts.

Section 27.2 Summary – pages Annelids probably evolved in the sea, perhaps from larvae of ancestral flatworms. Origins of Segmented Worms

Section 27.2 Summary – pages Tubes constructed by polychaetes are the most common fossils of this phylum. Some of these tubes appear in the fossil record as early as 540 million years ago. Origins of Segmented Worms