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