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) 1. Mouth – earthworm takes soil into its mouth, the beginning of the

digestive tract2. Crop – holds soil temporarily before passing into the gizzard3. Gizzard – grinds organic matter of food into small pieces so that the

nutrients in the food can be absorbed4. Circulatory System – closed system consist of enlarged blood vessels

that are heavily muscled. When these muscles contract they help pump blood through the system

5. 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

6. Nepheridia – excretory structures that eliminate metabolic waste7. 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