phylum annelida. what is a segmented worm? this phylum includes leeches, bristleworms, and...
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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.
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