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Annelids
Annelids – Metameric Coelomates
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Characteristics of Annelids:
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1: Repeated segments – Each segment has
digestive, excretory and locomotors organs
repeated
2: Specialized segments – anterior has special
segments with sensory organs
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Ex: Central nerve cord and Cerebral Ganglion (Brain)
3: Connection between segments – circulatory
system functions between segments.
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Each segment is separated by septa.
More Characteristics
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Coelomates
Segmentation
Organ systems – highly specialized gut, closed
circulatory system, nephridia for excretion
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Nephridia is used for ionic regulation, osmotic
regulation and metabolism (most dump ammonia
instead of urea)
More Characteristics
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Bristles – most annelids have external bristles
called setae or parapodia
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Many with specialized functions
 Used for : swimming, crawling, holding,
protection, respiration, and feeding.
Organ Systems
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Have a well defined
circulatory system for
pumping blood to all the
segments
Exchange oxygen and
carbon dioxide with the
environment through
the skin
Use ciliated nephridia to
expel waste
Closed system circulatory system
Three Classes
1.
2.
3.
Polychaetes
Oligochaetes
Hirudinean
Polychaetes
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Always have a parapodia (usually a pair on
each segment
Many setea (hair-like sensory projections
Almost entirely marine
Mostly diecious (separate sexes)
Fertilize externally (mostly)
Polychaetes
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Nutrient uptake:
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Some predators (free swimmers)
some feed on sediments (burrow in ground)
some filter feed (live at bottom of ocean)
Well developed head – have antennae,
specialized mouthparts and sense organs
(stalked eyes)
Usually very colorful
Trochophora larva
Oligochaetes
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Mostly terrestrial or fresh water
Mostly hermaphroditic involving mostly cross
fertilization
Few setae
No parapodia
No eyes
Lack Head region
Oligochaetes
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Are light sensitive and touch sensitive
Detect moisture
Will drown when rains (why?)
Earthworms aerate the soil and fertilize it
Clitallum – pouch-like receptacle on worm for
reproduction
Hirudninean
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Mostly freshwater (some terrestrial and marine)
Mostly hermaphroditic involving mostly cross
fertilization
Attachment suckers
Modified mouth and gut
No setae
No parapodia
Flattened bodies
Parasitic, scavengers, predators
Medical Leeches
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First known use was in ancient Egypt 2500 yrs
ago.
They are used still today in the aid of
reattachment of severed fingers and toes.
http://www.leechesusa.com/LEECHESUSA/ge
neral_information.asp
HW
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1. If you have two worms with different external
characteristics, how can you tell if either is an
annelid?
2. How are the circulatory systems of a human and
an earthworm similar?
3. Contrast polychaetes and leeches
4. A mutation results in the birth of an earthworm
that lacks moisture sensitive cells in its skin. Explain
why this earthworm is less likely to survive than one
with the sensory cells.