animal evolution –the invertebrates
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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 gut
fluid 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
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
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.
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
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
Animation: Earthworm body plan
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 organs
radula 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
Animation: Snail body plan
Animation: Torsion in gastropods
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 lung
sensory 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
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
Fig. 25-26d, p. 418
arm
beak
internal shellradula mantle
anus accessory heart
reproductive organ
tentaclesiphon gill
heartink sac
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
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
Animation: Blood fluke life-cycle
Animation: Feeding leech
Animation: Marine polychaetes
Animation: Molluscan classes
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