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27-3 Annelids
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What Is an Annelid?
What Is an Annelid?
Phylum Annelida
Annelids are worms with segmented bodies.
They have a true coelom that is lined with tissue derived from mesoderm.
Annelida is derived from Latin meaning “little ring,” referring to a ringlike appearance of annelids’ body segments.
Three Germ Layers
of an Annelid
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What Is an Annelid?
Each segment is separated by septum, which are internal walls between each segment.
Most segments are similar to one another, although they may be modified to perform special functions.
Body segments may carry eyes, antennae, other sense organs, or be specialized for functions such as respiration.
Bristles called setae may be attached the segments.
Annelids have a coelm that food passes through from the mouth to the anus.
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4. Form and Function in Annelids
Annelids have complex organ systems.
Many of these systems are unique because of the segmented body plan of this group.
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5. Form and Function in Annelids
Feeding and Digestion
Annelids range from filter feeders to predators.
In carnivorous species, the pharynx usually holds two or more sharp jaws that are used to attack prey.
Annelids that feed on decaying vegetation have a pharynx covered with sticky mucus.
Other annelids obtain nutrients by filter feeding, fanning water through tube-like burrows and catching food particles in a mucous bag.
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6. Form and Function in Annelids
In earthworms, the pharynx pumps food and soil into the esophagus.
The food then moves through the crop, where it can be stored.
It then moves through the gizzard, where it is ground into smaller pieces.
The food is absorbed farther along in the digestive tract in the intestine.
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7. Form and Function in Annelids
Circulation 
Annelids typically have a closed circulatory system, in which blood is contained within a network of blood vessels.
An earthworm’s blood circulates through two major blood vessels that run from head to tail.
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8. Form and Function in Annelids
Blood in the dorsal (top) vessel moves toward the head of the worm.
The dorsal blood vessel functions like a heart because it contracts rhythmically and helps pump blood.
Aortic arches act as accessory “hearts,” and are the pumping organs along with the dorsal blood vessel.
Dorsal blood vessel
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9. Form and Function in Annelids
Blood in the ventral (bottom) vessel runs from head to tail.
Ventral blood vessel
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10. Form and Function in Annelids
In each body segment, a pair of smaller blood vessels connect the dorsal and ventral blood vessels and supply blood to the internal organs.
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11. Form and Function in Annelids
Respiration 
Aquatic annelids often breathe through gills.
A gill is an organ specialized for the exchange of gases underwater.
Land-dwelling annelids take in oxygen and give off carbon dioxide through their moist skin.
They secrete a thin protective layer of mucus to keep their skin moist.
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12. Form and Function in Annelids
Excretion  
Digestive waste passes out through the anus.
Cellular waste containing nitrogen is eliminated by nephridia.
Nephridia are excretory organs that filter fluid in the coelom.
Nephridia
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13. Form and Function in Annelids
Response  
Most annelids have a well-developed nervous system consisting of a brain and several nerve cords.
Sense organs are best developed in free-living marine annelids.
Many species have adaptations for detecting stimuli including sensory tentacles, chemical receptors, statocysts, and two or more pairs of eyes.
Brain
Ganglia
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14. Form and Function in Annelids
Movement  
Annelids have two groups of body muscles that function as part of a hydrostatic skeleton.
Longitudinal muscles run from the front of the worm to the rear and can contract to make the worm shorter and fatter.
Circular muscles wrap around each body segment and can contract to make the worm longer and thinner.
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15. Form and Function in Annelids
The earthworm moves by alternately contracting these two sets of muscles, using setae to prevent slipping.
Marine annelids use paddle-like appendages called parapodia for swimming and crawling.
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16. Form and Function in Annelids
Reproduction   
Most annelids reproduce sexually.
Some species use external fertilization and have separate sexes.
Other annelids are hermaphrodites. Individuals rarely fertilize their own eggs.
Two worms attach to each other, exchange sperm, and then store the sperm in special sacs.
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17. Form and Function in Annelids
When eggs are ready for fertilization, a clitellum secretes a mucus ring into which eggs and sperm are released.
A clitellum is a band of thickened, specialized segments.
After eggs are fertilized in the ring, the ring slips off the worm's body and forms a protective cocoon.
Young worms hatch weeks later.
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Groups of Annelids
Groups of Annelids
Annelids are divided into three classes
oligochaetes:
leeches
polychaetes
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19. Class Oligochaeta: Oligochaetes
Groups of Annelids
Class Oligochaeta: Oligochaetes
Oligochaetes contains earthworms and their relatives.
They typically have streamlined bodies and relatively few setae compared to polychaetes.
Most oligochaetes live in soil or fresh water and do not have separate sexes.
Their pharynx sucks a mixture of detritus and soil into the mouth and as the mixture passes through the intestine, the food particles are digested and absorbed.
Sand, clay, and indigestible matter pass out through the anus in large quantities, producing castings.
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20. Class Hirudinea: Leeches
Groups of Annelids
Class Hirudinea: Leeches
Leeches are typically external parasites that suck the blood and body fluids of their host.
Most live in moist habitats in tropical countries and are hermaphroditic.
One fourth of all leeches are carnivores that feed on soft-bodied invertebrates (snails, worms, and larvae).
Leeches have powerful suckers at both ends of their bodies that help them cling to their hosts.
Some leeches force a muscular extension called a proboscis into the tissue of their host.
Others slice into the skin with razor-sharp jaws.
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Groups of Annelids
The leech uses its pharynx to suck blood from the wound.
Some leeches release a substance that anesthetizes the wound—keeping the host from knowing it has been bitten.
Leeches were once used to treat medical conditions, and are experiencing a medical revival of sorts today.
They help to reduce swelling after surgery because they secrete a fluid that prevents blood from clotting.
This anti-clotting mechanism helps relieve pressure and congestion in the healing tissues.
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22. Class Polychaeta: Polychaetes
Groups of Annelids
Class Polychaeta: Polychaetes
Polychaetes include sandworms, bloodworms, and their relatives.
They live in cracks and crevices in coral reefs; in sand, mud, and piles of rocks; or even in open water.
They have separate sexes.
Polychaetes are marine annelids that have paired, paddle-like appendages (parapodia) tipped with setae.
The setae are brush-like structures on the worm.
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Ecology of Annelids
Ecology of Annelids
Earthworms and many other annelids spend their lives burrowing through soil, aerating and mixing it.
Earthworms help plant matter decompose.
Earthworm castings are rich in nitrogen, phosphorus, potassium, micronutrients, and beneficial bacteria.
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