Download Ch. 27.3

Ch. 27.3

Found everywhere
 Soil, sea, parasites
Can look like flat or round worms
 Actually, more related to clams or octopi!
(Share a larval stage)

What is an Annelid?
Phylum: ANNELIDA; means ‘little ring’
 SEGMENTED

 Separated by SEPTA  internal walls
between each segment
Some segments carry sense organs
 Annelids have SETAE (bristles) on each
segment

Contain a true COELEM lined with a
mesoderm
 Tube within a tube digestive tract
(mouth  anus)

Form and Function in Annelids

Feeding and digestion: filter feeders or
predators
 Use a pharynx
 Can use mucus to break down decaying
materials
 Filter feeders

Crop and gizzard in earthworms (crop
stores; gizzard grinds it up)
Circulation
Closed circulatory system  blood
contained in a network of blood vessels
 Blood circulates in 2 major blood
vessels (head to tail).

 Blood in the dorsal end (top) goes
TOWARD the head
 Blood in the ventral end (bottom) goes
TOWARD the tail
Smaller blood vessels appear in each
segment and supply blood to internal
organs
 Dorsal blood vessel functions like a
heart, contracts rhythmically

Respiration
Aquatic annelids have GILLS
 Land annelids have gas exchange via
moist skin

 Secrete a thin protective coating of mucus
Excretion

Produce 2 kinds of waste
 Passes out via anus

Cellular waste that has nitrogen is
eliminated by NEPHRIDIA (filter)
Response
Well developed nervous system
consisting of a brain and several nerve
cords
 Marine annelids have other adaptations
(sensory tentacles, chemical receptors,
gravity sensors, eyes)

Movement
Body muscles part of the hydrostatic
skeleton
 Longitudinal muscles run from front of
worm to the rear; contraction
 Circular muscles wrap around each
body segment; contract to make worm
longer and thinner
 Alternate contracting

Reproduction
Most reproduce sexually
 External fertilization
 Separate sexes
 Some are hermaphrodites
 CLITELLUM  thickened, specialized
segments secrete mucus rings into
which sperm and eggs are released
(forms a protective cocoon)

Groups of Annelids

Oligochaetes  earthworms;
streamlined bodies and few setae
 Live in fresh water or soil

Leeches  tropical areas; moist
habitats
 External parasites
 carnivores
Polychaetes  sandworms,
bloodworms
 Marine
 Paired, paddlelike appendages
