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Animal Evolution –The
Invertebrates
Chapter 25 Part 2
25.6 Flatworms—Simple Organ Systems
Flatworms (phylum Platyhelminthes) have a
three-layer embryo that develops into an adult
with many organ systems but no coelom
Three main classes: turbellarians, flukes
(trematodes), and tapeworms (cestodes)
Turbellarians:
Structure of a Free-Living Flatworm
Pharynx
• Muscular tube connecting the mouth with the gut
Nerve cords
• Two lines of communication along length of body
Ganglia
• Cluster of nerve cell bodies (simple brain)
Flatworm Organ Systems
Fig. 25-15, p. 412
nucleus
rudimentary
brain (pair of
large ganglia
in head)
ovary
pair of
highly
branched
tubules that
adjust water
and solute
levels in
body
cilia
branching
gutfluid filters
through
membrane
folds
testis
oviductpharynx;
protrudes
onto
food, then
retracts into
the body
between
feedings
genital
pore
pair of nerve
cords that
have lateral
branchings
flame cell
opening
at body
surface
Parasites: Flukes and Tapeworms
In blood flukes (Schistosoma), reproduction
takes place in mammals – immature stages live
in intermediate hosts (snails)
A tapeworm body consists of proglottids –
repeating hermaphroditic body units that bud
from a region behind the scolex
Fluke Life Cycle: Schistosoma
Fig. 25-16, p. 413
A A fluke matures
and mates in a
human host.
F Larvae burrow into new
human host, enter intestinal
veins, and start a new cycle. B Fertilized eggs
exit host in feces.
E Fork-tailed, swimming larvae
develop and leave the snail.
C Eggs hatch as
ciliated larvae.
D Larvae burrow into an aquatic
snail and multiply asexually.
Fig. 25-16, p. 413
A A fluke matures
and mates in a
human host.
C Eggs hatch as
ciliated larvae.
D Larvae burrow into an aquatic
snail and multiply asexually. Stepped Art
B Fertilized eggs
exit host in feces.
E Fork-tailed, swimming larvae
develop and leave the snail.
F Larvae burrow into new
human host, enter intestinal
veins, and start a new cycle.
Beef Tapeworm Life Cycle
Fig. 25-17, p. 413
proglottids scolex
B A human, the
definitive host,
eats infected,
undercooked beef,
which is mainly
skeletal muscle.
A Larvae, each with inverted
scolex of future tapeworm,
become encysted in
intermediate host tissues
(e.g., skeletal muscle).
scolex
attached
to wall of
intestine
one
proglottid
D Inside each fertilized egg,
an embryonic, larval form
develops. Cattle may ingest
embryonated eggs or ripe
proglottids, and so become
intermediate hosts.
C Each sexually mature
proglottid has female
and male organs. Ripe
proglottids containing
fertilized eggs leave the
host in feces, which may
contaminate water and
vegetation.
Fig. 25-17, p. 413
B A human, the
definitive host,
eats infected,
undercooked beef,
which is mainly
skeletal muscle.
A Larvae, each with inverted
scolex of future tapeworm,
become encysted in
intermediate host tissues
(e.g., skeletal muscle).
proglottids scolex
scolex
attached
to wall of
intestine
one
proglottid C Each sexually mature
proglottid has female
and male organs. Ripe
proglottids containing
fertilized eggs leave the
host in feces, which may
contaminate water and
vegetation.
Stepped Art
D Inside each fertilized egg,
an embryonic, larval form
develops. Cattle may ingest
embryonated eggs or ripe
proglottids, and so become
intermediate hosts.
25.7 Annelids—Segmented Worms
Annelids (phylum Annelida) are bilateral worms
with a coelom and a segmented body; typically
with chaetae (chitin reinforced bristles)
Three main groups: marine worms (polychaetes),
oligochaetes (including earthworms), and leeches
Marine Polychaetes
Fig. 25-18a, p. 414
Fig. 25-18a, p. 414
―jaws‖
toothlike structures
pharynx (everted)
antenna
palp (food handling)
eyes
chemical-
sensing pit
parapod
tentacle
Fig. 25-18b, p. 414
Leeches – Bloodsuckers and Others
Leeches lack chaetae and have a sucker at
either end
Fig. 25-19a, p. 414
before
feeding
Fig. 25-19b, p. 414
after
feeding
Oligochaetes
Example: earthworms
• Exchange gases across body surfaces
• Have five hearts and a closed circulatory system
• Nephridia regulate coelomic fluid
• Nervous system of ganglia and nerve cords
• Hydrostatic skeleton
• Hermaphroditic
Earthworm Body Plan
Fig. 25-20a, p. 415
Fig. 25-20a, p. 415
anus
nephridiumdorsal blood
vessel
clitellumcoelom
intestinegizzard
cropesophagus
pharynx
gut
longitudinal
muscle
2 of 5
hearts
brainventral
nerve
cord
ventral
blood
vessel
mouthcircular
muscle
ventral
nerve
cord
Fig. 25-20b, p. 415
Fig. 25-20b, p. 415
anus
clitellum head
How Earthworms Move
Fig. 25-21, p. 415
bristles used in locomotion
25.8 Mollusks—Animals With a Mantle
Mollusks (phylum Mollusca)
• Bilaterally symmetrical with a reduced coelom
• Mantle covers internal organs, secretes a shell
• Feed using a hard radula
• Have a complete digestive tract
• Gills for respiration in aquatic species
Mollusk Diversity
Chitons
• Eight overlapping plates
Gastropods (snails, slugs)
• Undergo torsion during development
Bivalves (mussels, clams, oysters)
• Hinged, two-part shell
Cephalopods (squids, octopuses)
• Large, fast and smart; closed circulatory system
Mollusk Groups
Fig. 25-22a, p. 416
Fig. 25-22b, p. 416
Fig. 25-22c, p. 416
Fig. 25-22d, p. 416
Gastropod Body Plan
Fig. 25-23a, p. 416
Fig. 25-23a, p. 416
anus gill excretory organ
mantle
cavity
heart
digestive
gland
shell
stomach
edge of mantle
that covers organsradula foot
Fig. 25-23b, p. 416
Fig. 25-23b, p. 416
before
torsion:
mouth mouthafter
torsion:mantle’s edge anus
anus, which
discharges
wastes into
mantle cavity
Variations on the Gastropod Body Plan
Fig. 25-24a, p. 417
Fig. 25-24b, p. 417
Fig. 25-24b, p. 417
mantle eye
opening that
leads to lungsensory
tentacle
foot
Fig. 25-24c, p. 417
Bivalve Body Plan: Clam
Fig. 25-25, p. 417
mouth left mantle adductor muscle (cut)
adductor
muscle
(cut)
Water flows out through
exhalant siphon
Water flows in through
inhalant siphon
foot palps left gill shell
25.9 Cephalopods—Fast and Brainy
Cephalopod (“head foot”)
• Tentacles attached to the head are evolutionary
modifications of the foot; they surround the
mouth, which has a hard, horny beak
Include the fastest (squids), biggest (giant
squid), and smartest (octopuses) invertebrates
• Jet propulsion, complex eyes, closed circulatory
system, complex behavior
Cephalopods
Fig. 25-26a, p. 418
Fig. 25-26b, p. 418
Fig. 25-26c, p. 418
Fig. 25-26d, p. 418
Fig. 25-26d, p. 418
arm
beak
internal
shellradula mantle
anus accessory
heartreproductive
organtentacle
siphon gill
heartink
sac
Fig. 25-26e, p. 418
25.10 Rotifers and Tardigrades—
Tiny and Tough
Rotifers (phylum Rotifera) and tardigrades
(phylum Tardigrada) are tiny bilateral animals
Rotifers have a pseudocoelom, but are
genetically closest to annelids and mollusks
Tardigrades have a coelom and molt, and are
probably relatives of roundworms and insects
Rotifer Body Plan
Fig. 25-27, p. 419
ciliated lobe
mouth
brain
with
eyespots
protonephridium
stomach
intestine
anus
one of two
―toes‖
Tardigrades
Fig. 25-28a, p. 419
tardigrade’s
mouth
roundworm
prey
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