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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
branching
gut
rudimentary
brain (pair of
large ganglia
in head)
ovary
testis
oviduct
pharynx;
protrudes
onto
food, then
retracts into
the body
between
feedings
pair of nerve
cords that
have lateral
branchings
pair of
highly
branched
tubules that
adjust water
and solute
levels in
body
flame cell
nucleus
cilia
fluid filters
through
membrane
folds
genital
pore
opening
at body
surface
Fig. 25-15, p. 412
Animation: Planarian organ systems
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
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.
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.
Stepped Art
Fig. 25-16, p. 413
Beef Tapeworm Life Cycle
A Larvae, each with inverted
scolex of future tapeworm,
become encysted in
intermediate host tissues
(e.g., skeletal muscle).
proglottids
B A human, the
definitive host,
eats infected,
undercooked beef,
which is mainly
skeletal muscle.
scolex
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
A Larvae, each with inverted
scolex of future tapeworm,
become encysted in
intermediate host tissues
(e.g., skeletal muscle).
proglottids
B A human, the
definitive host,
eats infected,
undercooked beef,
which is mainly
skeletal muscle.
scolex
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.
Stepped Art
Fig. 25-17, p. 413
Animation: Tapeworm life cycle
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
“jaws”
toothlike structures
pharynx (everted)
antenna
palp (food handling)
tentacle
eyes
chemicalsensing pit
parapod
Fig. 25-18a, p. 414
Fig. 25-18b, p. 414
Leeches – Bloodsuckers and Others
 Leeches lack chaetae and have a sucker at
either end
before
feeding
Fig. 25-19a, p. 414
after
feeding
Fig. 25-19b, p. 414
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
anus
nephridium
dorsal blood
vessel
clitellum
coelom
intestine
gizzard
crop
esophagus
pharynx
gut
longitudinal
circular
muscle
muscle
ventral
nerve
cord
mouth
2 of 5
hearts
brain
ventral
nerve
cord
ventral
blood
vessel
Fig. 25-20a, p. 415
Fig. 25-20b, p. 415
anus
clitellum
head
Fig. 25-20b, p. 415
Animation: Earthworm body plan
How Earthworms Move
bristles used in locomotion
Fig. 25-21, p. 415
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
anus
gill
excretory organ
heart
mantle
cavity
digestive
gland
stomach
shell
edge of mantle
that covers organs
radula
foot
Fig. 25-23a, p. 416
Fig. 25-23b, p. 416
before
torsion:
mouth
mantle’s edge
after
torsion:
mouth
anus
anus, which
discharges
wastes into
mantle cavity
Fig. 25-23b, p. 416
Animation: Snail body plan
Animation: Torsion in gastropods
Variations on the Gastropod Body Plan
Fig. 25-24a, p. 417
Fig. 25-24b, p. 417
mantle
foot
opening that
leads to lung
eye
sensory
tentacle
Fig. 25-24b, p. 417
Fig. 25-24c, p. 417
Bivalve Body Plan: Clam
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
Fig. 25-25, p. 417
Animation: Clam body plan
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
arm
beak
radula
tentacle
internal
mantle shell
anus ink
heart accessory reproductive
sac gill
heart
organ
siphon
Fig. 25-26d, p. 418
Fig. 25-26e, p. 418
Animation: Cuttlefish body plan
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
ciliated lobe
mouth
brain
with
eyespots
protonephridium
stomach
intestine
anus
one of two
“toes”
Fig. 25-27, p. 419
Tardigrades
tardigrade’s
mouth
roundworm
prey
Fig. 25-28a, p. 419
Animation: Blood fluke life-cycle
Animation: Feeding leech
Animation: Marine polychaetes
Animation: Molluscan classes