Mollusks and AnnelidsBio IIRupp

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Phylum Mollusca Characteristics• Mollusk—from the Latin

meaning “soft”• Many have hard shells for

protection• Approx. 112,000 species• Range from sedentary

feeders to fast moving predators

• Coelomates▫ Muscles can move

without affecting gut▫ Circulatory system is not

interfered with• Trochophore—larval stage

▫ Free-swimming▫ Mouth and anus▫ Cilia for movement and

food gathering▫ Annelids and mollusks

both have, which shows common ancestry

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Trochophore

Mollusk and annelid larval stage

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Mollusk Body Plan

•Two main regions▫Head-foot

Contains sensory organs in head Foot contains muscle for locomotion

▫Visceral mass Contains heart, digestive organs, excretion

organs, and reproduction organs

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Mollusk Body Plan con’t.• Coelom around heart• Mantle covering the

visceral mass—aka—epidermal layer

• Mantle secretes CaCO3

▫ Shells protect, but decrease surface area

▫ Gills evolved▫ Gills are protected by

mantle cavity

• Bilateral symmetry—apparent in nervous system

• Ganglia are connected by nerve cords▫ Light▫ Touch▫ Chemicals

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Mollusk Body Plan con’t.

•Radula▫Main feeding adaptation▫Tongue-like strip covered in backwards

facing teeth▫Adapted to mollusk lifestyle

Terrestrial snails eat grass Aquatic snails eat algae or drill shells Cone shell has a harpoon-like radula

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Mollusk Body Plan con’t.• There are 8 classes, we

will focus on 3▫ Gastropoda▫ Bivalvia▫ Cephalopoda

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Table 37-1Feature Gastropoda Bivalvia Cephalopoda

External shell One in most species, none in slugs and nudibranchs

Two None except chambered nautilus

Head Yes No Yes

Radula Yes No Yes

Locomotion Most crawl Most are sessile Rapid swimming

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Class Gastropoda• Gastro means “stomach”

and poda means “foot”• 90,000 species• Snails, abalones, conchs—

one shell• Slugs and nudibranchs—

no shell

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Class Gastropoda con’t.• Torsion—the twisting

process that relocates the mantle cavity▫ Occurs in larval

development▫ Places mantle cavity near

the head▫ Allows head to be

retracted▫ Coiling of the shell is

unrelated

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Class Gastropoda con’t.• Movement occurs by

wave-like contractions of the foot

• Open circulatory system▫ Hemolymph▫ Hemocoel—blood cavity

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Snails• Terrestrial, marine, and

freshwater• Eyes on the end of

tentacles—they can retreat into head

• Gill-breathers• Land snails have modified

gills for gas exchange▫ Need to stay moist▫ Dormancy—mucous plug

—prevents desiccation

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Slugs• Respire through modified

mantle cavity• Live in moist shady places

to prevent desiccation

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Nudibranchs• Marine• No shell• Name means “naked gill”• Respiration occurs across

the entire body• Numerous extensions

create greater surface area

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Pteropods• Name means “wing foot”• Use the foot to swim

instead of crawl

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Class Bivalvia• Name means “two shells”• Clams, oysters, scallops• Adductor muscles for

closing their shells▫ Contraction closes▫ Relaxation opens

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Class Bivalvia con’t.• Each valve or shell has

three layers▫ Thin outer layer to

protect against acidity of sea water

▫ Thick middle layer of CaCO3

▫ Inner layer has “mother of pearl” coating to create a smooth surface for the soft body

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Class Bivalvia con’t.• Typically sessile

▫ Use their foot as an anchor

▫ Fill it with hemolymph to create a hatchet shape

▫ Contract foot and pull themselves into the sand

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Class Bivalvia con’t.• Filter feeders• No radula• No distinct head• Nervous system has three

ganglia pairs▫ Mouth▫ Digestive system▫ Foot▫ All are connected

• Respond to chemicals and touch

• Some have eyes along mantle—calico scallop

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Clams• Live buried in mud or

sand• Mantle cavity is sealed

except for the siphons▫ Cilia generate current to

pull water in▫ Water exits through

excurrent siphon• Gills capture organic

matter in mucous

• Gills are also used for gas exchange with hemolymph

• Typically separate sexes• Marine clams use external

fertilization• Freshwater clams use

internal fertilization▫ Sperm in incurrent

siphon▫ Larvae exit the excurrent

siphon▫ Eventually settle on

bottom and mature

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Oysters• Make permanent

attachment to hard surface early in life

• Commercially grown for food and pearls

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Scallops• Motile• Use shells in a clapping

motion to swim

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Teredo or Shipworm• Not a filter feeder• Bores into driftwood or

boat timbers and ingests the particles

• Wood is broken down by symbiotic bacteria

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Class Cephalopoda• Name means “head foot”• Octopuses, cuttlefish,

squids, chambered nautliluses

• Foot is concentrated in the head, hence the name

• Free-swimming

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Class Cephalopoda con’t.• Predatory lifestyle• Circle of tentacles

extending from the head• Jaws resembling a parrot’s

beak• Advanced nervous system

▫ Largest brain of any invertebrate

▫ Divided into lobes▫ Millions of cells▫ Capable of learning and

problem solving

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Class Cephalopoda con’t.• Well developed senses

▫ Eyes can form images▫ Tentacles have chemical

sensing ability• Closed circulatory system• Separate sexes

▫ Males have specialized tentacles for sperm transfer

▫ Females lay and guard eggs

▫ No trochophore stage

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Squids• Ten tentacles• Two longest tentacles for

prey capture, other eight force food into mouth

• Pump water through excurrent siphon for propulsion

• Most grow to 1 ft long• Giant squid grows to 60 ft

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Octopuses• Eight tentacles• Don’t chase prey like

squid—crawl and grab• Seldom exceed 3 ft in

diameter• Pacific octopus can grow

to 8 ft in diameter

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Chambered Nautilus• Shell• Series of gas-filled

chambers separated by partitions

• Confined to outermost chamber

• Gas in the chambers allows buoyancy

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Cuttlefish

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Phylum Annelida Characteristics• Name means “little rings”• 15,000 species• Bilaterally symmetrical• Most distinctive feature is

the rings• True coelom

▫ Divided into compartments by partitions

▫ Evolutionary advantage—allows different parts to expand and contract independently

• Duplication of organ systems in each segment—injury insurance

• Setae and parapodia—allow for division into three classes▫ Oligochaeta▫ Polychaeta▫ Hirudinea

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Class Oligochaeta• Name means “few

bristles”• Live in soil or freshwater• No parapodia• Have a few setae on each

segment• Most familiar class

member is the earthworm

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Class Polychaeta• 2/3’s of annelids• Name means “many

bristles”• Setae are found on

parapodia• Antennae and specialized

mouth parts• Only annelids with

trochophore stage• Most are marine• Some are free-swimming

predators and use strong jaws for feeding

• Others feed on sediment

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Class Hirudinea• 300 species• Live in calm bodies of

freshwater or moist vegetation on land

• No setae or parapodia• Each end has a sucker• Can crawl using suckers• Can swim with undulating

motion• Carnivorous

▫ Invertebrate prey▫ Blood suckers▫ Secrete anesthetic and

anticoagulant▫ Can ingest 10x their

weight

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Earthworm Structure and Movement• More than 100 segments• Circular and longitudinal

muscles• Movement

▫ Anchor posterior setae▫ Contract circular muscles

to extend▫ Anchor anterior setae▫ Contract longitudinal

muscles to pull forward

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Earthworm Feeding and Digestion• Ingest soil while burrowing

—use muscular pharynx• Soil moves into the crop—

temporary storage area• Soil is moved from crop to

gizzard—a muscular gut that grinds the soil to release the nutrients

• Nutrients are absorbed by the intestine—typhlosole folds

• Undigested materials exit through anus

• Maintains soils fertility by decomposition and aeration

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Earthworm Circulation• Closed system• Blood travels to the

posterior through a ventral blood vessel

• Blood returns to the anterior heart through a dorsal blood vessel

• Aortic arches link the vessels—these arches act as hearts to contract and move the blood

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Earthworm Respiration and Excretion

• Gases are exchanged by diffusion through skin and cuticle—no specialized organs

• Diffusion only occurs if skin is moist—mucous and cuticle aid in this effort

• Cell wastes are eliminated through nephridia▫ Each segment, except, the

first three, and last one, have nephridia

▫ Coelomic fluid passes through, water is reabsorbed, wastes eliminated

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Earthworm Nervous System• Chain of ganglia connected

by a ventral nerve cord• Most segments contain a

ganglion• Nerves branch from ganglia• Several anterior ganglia are

fused to form a brain▫ Light▫ Touch▫ Chemicals▫ Moisture▫ Temperature▫ Vibration

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Earthworm Reproduction• Hermaphrodites that do not

self-fertilize• Mating

▫ Press ventral surfaces together

▫ Anterior ends are pointed in opposite directions

▫ Held together by mucous and setae at the clitellum

▫ Sperm is injected and moves to seminal receptacle

▫ Each worm then secretes a tube of mucous and chitin where sperm and eggs meet outside the body

• 2-3 week development prior to hatching

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