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

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 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

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 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

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

heartreproductive

organtentacle

siphon 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