Chapter 27 Mollusks and Segmented Worms

27.2 Segmented Worms What is a Segmented Worm? Phylum Annelida Earthworms, leeches and bristle worms Bilaterally symmetrical, have a coelom and 2 body openings Some have a larval stage similar to those in mollusks Basic plan is a tube within a tube Most have tiny bristles called setae (help in movement) Found in most environments

27.2 Segmented Worms Segmentation Supports Diversified Functions Most distinct feature is their cylindrical bodies that are divided into a series of ringed segments. Each segment continues internally Segments are an important adaptment for moving because of the muscles in each segment.

27.2 Segmented Worms Nervous System in Segmented Worms Simple nervous systems (organs in anterior end have become modified for sensing the environment) Some sensory organs are sensitive to light Nerve cords connect the brain to nerve centers called ganglia, located in each segment.

27.2 Segmented Worms Circulation and Respiration Segmented worms have a closed circulatory system Blood carrying oxygen to and carbon dioxide from body cells, flows through vessels to reach all parts of the body. Must live in water or moist soil because they exchange gasses through their moist skin.

27.2 Segmented Worms Digestion and Secretion Have a complete internal digestive tract that runs the length of the body Food passes through the gizzard (grinds soil before the soil passes into the intestine) Solid waste pass through the anus Have 2 Nepheridia in each segment, collects waste and transport them through the coelom and out of the body)

27.2 Segmented Worms Reproduction in Segmented Worms Most are hermaphrodites During mating 2 worms exchange sperm Each worm forms a capsule for eggs and sperm The eggs are fertilized within this capsule and then the capsule is left behind by the worm in soil In 2 – 3 weeks worms emerge (Earthworms and leeches both do this) Bristle worms and their relatives have separate sexes and reproduce sexually .

27.2 Segmented Worms Diversity of Segmented Worms Phylum Annelida is divided into 3 Classes Oligochaeta (Earthworms) Polychaeta (Bristle worms and their relatives) Hirudinea (Leeches)

27.2 Segmented Worms Diversity of Segmented Worms (Earthworms) Most well known annelid Do not have a distinct head Have few setae on each segment Eat their way through soil Help aerate soil Waste is called castings (help fertilize soil)

27.2 Segmented Worms Diversity of Segmented Worms (Earthworms)

27.2 Segmented Worms Diversity of Segmented Worms (Earthworms) Mouth – earthworm takes soil into its mouth, the beginning of the digestive tract Crop – holds soil temporarily before passing into the gizzard Gizzard – grinds organic matter of food into small pieces so that the nutrients in the food can be absorbed Circulatory System – closed system consist of enlarged blood vessels that are heavily muscled. When these muscles contract they help pump blood through the system Nervous System – a system of nerve fibers in each segment, coordinated by a simple brain that is above the mouth , also has a ventral nerve cord Nepheridia – excretory structures that eliminate metabolic waste Setae – protrude during muscle contraction and anchor the earthworm to the soil.

27.2 Segmented Worms Diversity of Segmented Worms (Bristleworms) Primarily marine organisms Each segment contains many setae giving the worm its name (Bristle) Has a head with well developed sense organs

27.2 Segmented Worms Diversity of Segmented Worms (Leeches) Segmented worms with flattened bodies and no setae. Most live in freshwater Most are parasites