1. Chapter 27 Mollusks and Segmented Worms

2. 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

3. 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.

4. 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.

5. 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.

6. 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)

7. 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 .

8. 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)

9. 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)

10. 27.2 Segmented Worms
Diversity of Segmented Worms (Earthworms)

11. 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.

12. 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

13. 27.2 Segmented Worms Diversity of Segmented Worms (Leeches)
Segmented worms with flattened bodies and no setae.
Most live in freshwater
Most are parasites