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Worms
Chapter 27
Flatworm characteristics
• Soft, flattened bodies
• Simplest animals with three embryonic
germ layers
• Bilateral symmetry
• Cephalization: head formation
• Acoelomate: fluid filled body cavity made
of mesoderm tissue
Flatworm feeding styles
• Carnivorous: feed on tiny aquatic animals
• Scavengers: feed on recently dead
animals
• Parasites: blood, tissue fluids, cells within
host’s body
– Lack complex digestive system because host
partially digests food
– Ex: tapeworm (no digestive system at all)
Feeding
• Mouth only
• Pharynx:
muscular tube,
extended from
mouth
• Digested food
and nutrients
absorbed in
the digestive
cavity
Branched digestive system
translucent species Aquaplana/Paraplanocera sp. extends throughout the entire body
Photo by Robert F. Bolland).
Respiration, Circulation, & Excretion
• Flat and thin body shape requires little
circulation.
• Diffusion transports oxygen and nutrients
to internal tissues
• Diffusion removes CO2 and waste
• Flame cells: specialized cells that remove
excess water from body
– helps to remove metabolic waste (ammonia
and urea)
Flame-bulb
excretory
system
Response
• Ganglia: group of nerve cells with two
long nerve cords
• Eyespots: group of cells that can detect
changes in light intensity
The Anatomy of a Flatworm
Section 27-1
Eyespot
Ganglia
Head
Nerve
cords
Gastrovascular
cavity
Flatworms use a pharynx to suck
food into the gastrovascular
cavity. Digested food diffuses
from the cavity into other cells of
the body. Eyespots in some
species detect light.
Mouth
Excretory
system
Freshwater flatworms have
simple ganglia and nerve
cords that run the length of
the body. The excretory
system consists of a network
of tubules connected to flame
cells that remove excess
water and cell wastes.
Ovary
Testes
Pharynx
Most flatworms are
hermaphrodites, having male
reproductive organs (testes)
and female reproductive
organs (ovaries) in the same
organism.
Flame cell
Excretory
tubule
Figure 27–3
Planarian with
noticeable eyespots
Eyespots of a planarian
Movement
• Cilia on epidermal cells
• Nerve controlled muscle cells
Muscle filament
staining of a
turbellarian flatworm
reveals a meshwork of
longitudinal, circular
and diagonal muscles.
The large, bright ring
with muscle fibers
radiating outward is
the muscular pharynx
plicatus
Image by M. D. Hooge
and S. Tyler,
Department of
Biological Sciences,
University of Maine).
Groups of flatworms
• Turbellarians: free living, marine &
freshwater
• Flukes: parasitic, infects internal organs of
host
• Tapeworms: parasitic, adapted to life
inside host’s intestines
Turbellarian
Brightly colored turbellarian
Turbellarian reproduction
Asexual - fission
Section 27-1
Life cycle of fluke
Flukes mature and reproduce
sexually in the blood vessels
of human intestines.
Embryos are released and
passed out with feces.
Primary host
(human)
Intermediate host
(snail)
Human
intestine
Adult
fluke
Embryo
Ciliated
larva
Tailed
larva
After asexual
reproduction, new
larvae are released
from the snail into the water.
They then infect humans,
the primary host, by
burrowing through the skin.
If they get into the water,
embryos develop into
swimming larvae that infect
an intermediate host (snail).
Species: Schistosoma Mansoni
Brachylaima cribbi fluke
• (A) adult worm
• (B) two suckers and
spiny coat
• (C) sperm deposited
from pore
Tapeworm
Proglottid: tapeworm segment
• Proglottid with fertilized eggs bursts open
to release zygotes
Human tapeworm
• Scolex: head with
suckers or hooks
Parasitic tapeworm from fish
Intermediate hosts
• Tapeworm larvae burrow into cow or fish
muscle tissue, which infects the animal
that eats this tissue.
Rat tapeworm
Roundworms
C. elegans
Roundworm characteristics
• Unsegmented
• Tapering ends
• Variety of
environments and
climates
Roundworm characteristics
• Pseudocoelomate
: body cavity lined
partially with
mesoderm tissue
• Two digestive tract
openings, mouth
and anus
Length of roundworm
• Microscopic
to 1 m in
length
Feeding
• Predators with
grasping
mouthparts
• Spines to catch
and eat small
animals
• Detritovores
parasitic hookworm
Respiration, Circulation, & Excretion
• Gas exchange through body walls
• Metabolic waste excretion through body
walls
• No internal transport system
Response and movement
• Simple nervous system of ganglia
• Hydrostatic pressure to move around
• Muscular contractions to move through
water
Reproduction
• Sexually
• Male or female reproductive system
• Internal fertilization: male deposits sperm
into female
• Complex life cycles in parasitic
roundworms
Guinea worm disease
Pinworm infection in child’s
intestine
Trichinella life cycle
Filarial worms
• Primary cause of
elephantiasis
condition where parts
of the body are
swollen
• Requires arthropod
intermediate host to
complete life cycle
Ascarid worms
Hookworm life cycle
C. elegans and research
• First multicellular
animal to have entire
genome sequenced
• Every body cell’s
developmental
pathway is also
recorded.
Segmented
worms
Annelid characteristics
• Segmented bodies
• True coelom: body cavity lined with
mesoderm tissue
• Mouth and anus
The Anatomy of an Earthworm
Section 27-3
Anus
Setae
Body segments
Gizzard Crop
Dorsal
blood vessel
Clitellum
Mouth
Brain
Ganglion
Circular muscle
Longitudinal
muscle
Nephridia
Ganglia
Ring
vessels
Figure 27–16
Reproductive
organs
Ventral
blood vessel
Feeding and Digestion
• Filter feeders
• Predators
• Pharynx that may be collect or capture (if
jawed) food
• Food moved into esophagus, crop for
storage, gizzard for grinding, digestive
tract for absorption
Circulation
• Closed circulatory system: blood
contained within network of blood vessels
• Two major blood vessels running from
head to tail
– Dorsal vessel: blood towards head, functions
like heart
– Ventral vessel: blood towards tail
Respiration
• Gills: specialized for underwater gas
exchange in aquatic worms (ex: tube
worms)
• Terrestrial annelids perform gas exchange
through their moist skin.
Tubeworms
Excretion
• Nephridia: organs that filter fluid in the
coelom
Response
• Most have well-developed nervous
systems with brain and nerve cords
• Marine annelids have the best adapted
nervous system.
Movement
• Marine annelids may have parapodia,
paddlelike appendages.
• Hydrostatic skeleton: longitudinal and
circular muscles
– Longitudinal: contract to make worm shorter
and fatter
– Circular: contract to make worm longer and
thinner
Reproduction
•
•
•
•
Sexual reproduction
External fertilization
May be single sex or hermaphrodite
Clitellum band thickens, secreting mucus
ring with eggs where sperm are deposited
• Fertilized eggs slips off with the mucus
ring, which forms a cocoon where the
young worms hatch later.
Clitellum
Earthworm cocoon
Oligochaetes: streamlined bodies,
few setae
Oligochaete worm with filamentous algae
Groups of annelids
• Leeches: external parasites that suck
blood and body fluids from host
Medical
leech used
in surgeries
Leeches release a chemical that
prevents the prey from forming
scabs. A leech bite is less
painful than it is just messy with
the profuse bleeding.
Polychaetes
• Marine annelids with paired, paddlelike
appendages with setae
Earthworms help compost matter.