Mollusca

“Soft Body”

Mollusca Characteristics

Unsegmented Coelomates: true body cavity

Bilateral Symmetry: divided lengthwise

Radula: cuticular band of teeth in the esophagus used for feeding (lost in bivalves)

Mollusca Body

Mantel: dorsal epithelium forms mantel which secretes calcareous spicules or one or more shells

Visceral Mass: contains mollusks organs

Foot: ventral body wall muscles develop into a locomotory or clinging foot

Mollusca Body

Mollusca Systems

Skeletal- Mantle may secretes a shell. Use hydrostatic pressure for ventral muscular foot.

Muscles -Ventral muscular foot and other muscles present.

Digestive- complete complex with salivary glands, digestive gland, and rasping tongue called a Radula.

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Mollusca Systems

Circulatory - Open except for Cephalopoda. Dorsal heart, usually in a pericardial cavity.

Respiratory - Ctenidia (gills) in mantle cavity, respiratory pigment is copper.

Excretory- by nephridia usually connecting to the pericardial cavity, the coelom is usually reduced to the cavities of the nephridia, gonads and pericardium.

Mollusca Systems

Nervous - Nerve ring with various pairs of ganglia—two pairs of nerve cords, one innervating the foot, the other the visceral mass (modified ventral ladder-like system)

Integumentary - Mantle

Endocrine - nervous systems produces hormones.

Reproductive - varied- monoecious, protandric, or dioecious. Larva in marine = trochophore and veliger, in freshwater clam is glochidium.

Mollusca

Body Plan

Mollusc Body Plan

Head-Foot Region

• Most molluscs have well developed head ends with sensory structures including photosensory receptors that may be simple light detectors or complex eyes (cephalopods).

Head-Foot Region

• The radula is a rasping, protrusible feeding structure found in most molluscs (not bivalves).
• Ribbon-like membrane with rows of tiny teeth.

Head-Foot Region

• The foot of a mollusc may be adapted for locomotion, attachment, or both.
• Pelagic forms may have a foot modified into wing-like parapodia.

Mantle Cavity

• The space between the mantle and the visceral mass is called the mantle cavity.
• The respiratory organs (gills or lungs) are generally housed here.

Shells

• When present, the calcareous shell is secreted by the mantle and is lined by it. It has 3 layers:
• Periostracum – outer organic layer helps to protect inner layers from boring organisms.
• Prismatic layer – densely packed prisms of calcium carbonate.
• Nacreous layer – iridescent lining secreted continuously by the mantle – surrounds foreign objects to form pearls in some.

Visceral Mass Function

• Many molluscs have an open circulatory system with a pumping heart, blood vessels and blood sinuses.
• Most cephalopods have a closed circulatory system with a heart, blood vessels and capillaries.

Visceral Mass Function

• Reproductive Structures:
• Divided into males and females
• Gonads are in Visceral Mass

Mollusc Life Cycle

• Most molluscs are dioecious, some are hermaphroditic.
• The life cycle of many molluscs includes a free swimming, ciliated larval stage called a trochophore.
• Similar to annelid larvae.

Mollusc Life Cycle

• The trochophore larval stage is followed by a free-swimming veliger larva in most species.

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Mollusca

Classes

Phylum Mollusca

Classes

Polyplacophora   Aplacophora   Monoplacophora   Scaphopoda

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Gastropoda              Bivalvia             Cephalopoda

• Phylum Mollusca includes snails and slugs, oysters and clams, and octopuses and squids.

Phylogeny

• The first molluscs probably arose during Precambrian times.
• Diverse molluscs found in the early Cambrian.
• It is likely that molluscs split off from the line that led to annelids after coelom formation, but before segmentation appeared.

Phylogeny

• “Hypothetical Ancestral Mollusc”
• Probably lacked a shell or crawling foot.
• Probably small (about 1 mm).
• Likely was a worm-like organism with a ventral gliding surface.
• Probably possessed a dorsal mantle, a chitinous cuticle and calcareous scales.

Phylogeny

• The question remains, did segmentation originate independently within the three metameric taxa?
• Ongoing studies suggest that differences in biochemical pathways and developmental steps that produce segmented bodies across taxa, support the hypothesis that segmentation arose several times independently.

Major Mollusc Classes

• Four major classes of molluscs:
• Class Polyplacophora – the chitons
• Class Gastropoda – snails & slugs
• Class Bivalvia – clams, mussels, oysters
• Class Cephalopoda – octopus & squid

Class Polyplacophora

• Class Polyplacophora includes the chitons.
• Eight articulated plates or valves.
• Can roll up.
• Live mostly in the rocky intertidal.
• Use radula to scrape algae off rocks.
• Gills are suspended from roof of mantle cavity.
• Water flows from anterior to posterior.

Class Polyplacophora

• Pair of osphradia serves as sense organ.
• Light sensitive esthetes form eyes in some species – pierce plates.
• Blood pumped by a three-chambered heart.
• Travels through aorta and sinuses to gills.
• Pair of metanephridia carries wastes from pericardial cavity to exterior.
• Sexes are separate.
• Trochophore larvae metamorphose into juveniles without a veliger stage.

Class Polyplacophora

Class Scaphopoda

• Class Scaphopoda includes the tusk shells.
• Found in subtidal zone to 6000 m deep.
• Mantle wraps around visceral mass and is fused, forming a tube.

Class Gastropoda

• Gastropoda is the largest of the molluscan classes.
• 70,000 named species.
• Include snails, slugs, sea hares, sea slugs, sea butterflies.
• Marine, freshwater, terrestrial.
• Benthic or pelagic

Class Gastropoda

• Gastropods show bilateral symmetry, but due to a twisting process called torsion that occurs during the veliger larval stage, the visceral mass is asymmetrical.

Class Gastropoda

• The shell of a gastropod is always one piece – univalve – and may be coiled or uncoiled.
• The apex contains the oldest and smallest whorl.
• Shells may coil to the right or left – this is genetically controlled.

Class Gastropoda

• Coiling is not the same as torsion.
• Early gastropods had a planospiral shell where each whorl lies outside the others.
• Bulky
• Conispiral shells have each whorl to the side of the preceding one.
• Unbalanced
• Shell shifts over for better weight distribution.

Class Gastropoda

• Many snails can withdraw into the shell and close it off with a horny operculum.

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Gastropod Feeding Habits

• Most gastropods are herbivores and feed by scraping algae off hard surfaces using the radula.
• Some are scavengers of dead organisms, again tearing off pieces with radular teeth.

Gastropod Feeding Habits

• Some are carnivores and have a radula modified into a drill to bore through the shells of other molluscs. They use chemicals to soften the shell.

Gastropod Feeding Habits

• Snails in the genus Conus feed on fish, worms, and molluscs.
• Highly modified radula used for prey capture.
• They secrete a toxin that paralyzes their prey.
• Some are painful, even lethal, to humans.

Gastropod Feeding Habits

• Flamingo tongue snails feed on gorgonians.
• Mantle is brightly colored and envelops the shell.

Internal Form and Function

• Respiration in many performed by ctenidia in mantle cavity.
• Derived prosobranchs lost one gill and half of remaining gill.
• Resulting attachment to wall of mantle cavity provided respiratory efficiency.

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Internal Form and Function

• Pulmonates lack gills.
• Have a highly vascular area in mantle that serves as lung.
• Lung opens to outside by small opening, the pneumostome.
• Aquatic pulmonates surface to expel a gas bubble and inhale by curling, thus forming a siphon.

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Internal Form and Function

• Most have a single nephridium and well-developed circulatory and nervous systems.
• Sense organs include eyes, statocysts, tactile organs, and chemoreceptors.
• Eyes vary from simple cups holding photoreceptors to a complex eye with a lens and cornea.
• Sensory osphradium at base of the incurrent siphon may be chemosensory or mechanoreceptive.

Internal Form and Function

• Monoecious and dioecious species.
• Young may emerge as veliger larvae or pass this stage inside the egg.
• Some species, including most freshwater snails, are ovoviviparous.

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Class Gastropoda

• Traditional classification has recognized three subclasses of Gastropoda:
• Prosobranchia, Opisthobranchia, and Pulmonata.

Class Gastropoda

• Prosobranchia includes most marine snails and some freshwater and terrestrial gastropods.
• Mantle cavity is anterior due to torsion.
• Long siphons may separate incurrent and excurrent flow.
• Have one pair of tentacles, separate sexes, and usually an operculum.

Class Gastropoda

• Opisthobranchia includes sea slugs, sea hares, sea butterflies, and canoe shells.
• Most are marine, shallow-water.
• Partial to complete detorsion - anus and gill(s) are displaced to right side.

Class Gastropoda

• Opisthobranchia
• Two pairs of tentacles, one pair modified to increase chemo-absorption.
• Shell is reduced or absent.
• Monoecious
• Sea hare Aplysia has large anterior tentacles and a vestigial shell.

Class Gastropoda

• Pulmonata includes land and most freshwater snails and slugs.
• Ancestral ctenidia have been lost and the vascularized mantle wall is now a lung.
• Air fills lung by contraction of mantle floor.

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Class Gastropoda

• Pulmonata
• Anus and nephridiopore open near the pneumostome.
• Waste is forcibly expelled.
• Monoecious
• Aquatic species have one pair of tentacles.
• Landforms have two pair of tentacles and the posterior pair has eyes.

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Class Bivalvia

• Bivalved molluscs have two shells (valves).
• Mussels, clams, oysters, scallops, shipworms.
• Mostly sessile filter feeders.
• No head or radula.

Class Bivalvia

• Bivalves are laterally (right-left) compressed and their two shells are held together by a hinge ligament on the dorsal surface.
• The Umbo is the oldest part of the shell, growth occurs in concentric rings around it.

Class Bivalvia

• Part of the mantle is modified to form incurrent and excurrent siphons.
• Used to pump water through the organism for gas exchange and filter feeding.
• Sometimes used for jet propulsion.

Class Bivalvia

• Shipworms can be destructive to wharves & ships.
• The valves have tiny teeth that act as wood rasps and allow these bivalves to burrow through wood.
• They feed on wood particles with the help of symbiotic bacteria that produce cellulase and fix nitrogen.

Class Bivalvia

• Native freshwater clams in the U.S. are jeopardized.
• Of more than 300 species once present, 12 are extinct, 42 are threatened or endangered and 88 more are of concern.
• Sensitive to water quality changes, including pollution and sedimentation.
• Zebra mussels are a serious exotic invader into the Great Lakes Region.

Class Bivalvia - Locomotion

• Bivalves move around by extending the muscular foot between the shells.
• Scallops and file shells swim by clapping their shells together to create jet propulsion.

Class Bivalvia

• Like other molluscs, bivalves have a coelom and an open circulatory system.
• The mantle cavity of a bivalve contains gills that are used for feeding as well as gas exchange.

Class Bivalvia

• Scallops have a row of small blue eyes along the mantle edge. Each eye has a cornea, lens, retina, and pigmented layer.

Class Bivalvia

• Pair of U-shaped kidneys is ventral and posterior to heart.
• Nervous system has three pairs of widely separated ganglia connected together.
• Sense organs are poorly developed.
• Statocysts in the foot.
• Osphradia in the mantle cavity (chemoreceptive).
• Pigment cells on the mantle.

Class Bivalvia

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• Some mantle eyes have a cornea, lens, retina and pigmented layer.
• Tentacles may have tactile and chemoreceptor cells.

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Class Bivalvia - Feeding

• Suspended organic matter enters incurrent siphon.
• Gland cells on gills and labial palps secrete mucus to entangle particles.
• Food in mucous masses slides to food grooves at lower edge of gills.
• Cilia and grooves on the labial palps direct the mucous mass into mouth.
• Some bivalves feed on deposits in sand.

Class Bivalvia - Reproduction

• Bivalves usually have separate sexes.
• Zygotes develop into trochophore, veliger, and spat (tiny bivalve) stages.

Class Bivalvia - Reproduction

• In freshwater clams, fertilized eggs develop into glochidium larvae which is a specialized veliger.
• Glochidia live as parasites on fish and then drop off to complete their development.

Class Cephalopoda

• Cephalopods (head foot) include octopuses, squid, nautiluses and cuttlefish.
• Marine carnivores with beak-like jaws surrounded by tentacles of their modified foot.
• Modified foot is a funnel for expelling water from the mantle cavity.

Class Cephalopoda

• Cephalopod fossils go back to Cambrian (570 mya) times.
• The earliest had straight cone-shaped shells.
• Later examples had coiled shells similar to Nautilus.
• Ammonoids were a very successful group, some had quite elaborate shells.

Class Cephalopoda - Shells

• Shells of Nautilus and early nautiloid and ammonoid cephalopods were made buoyant by a series of gas chambers.

Class Cephalopoda - Shells

• Nautilus shells differ from those of a gastropod because they are divided into chambers. The animal lives in the last chamber. A cord of living tissue extends through each chamber.

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Class Cephalopoda - Shells

• Cuttlefishes have a small curved shell, completely enclosed by the mantle.

Class Cephalopoda - Shells

• In squid, the shell has been reduced to a small strip called the pen, which is enclosed in the mantle.

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Class Cephalopoda - Locomotion

• Cephalopods swim by expelling water from the mantle cavity through a ventral funnel.
• They can aim the funnel to control the direction they are swimming.

Class Cephalopoda

• Cephalopods have a closed circulatory system.
• Nervous and sensory systems are more elaborate in cephalopods than in other molluscs.
• The brain is the largest of any invertebrate.

Class Cephalopoda

• Most cephalopods have complex eyes with cornea, lens, chambers, and retina.

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Class Cephalopoda – Communication

• Visual signals allow cephalopods to communicate.
• Movement of body and arms
• Color changes effected by chromatophores (cells in the skin containing pigment granules).
• Chromatophores can change shape alternately dispersing and concentrating pigment.

Class Cephalopoda

• Most cephalopods have an ink sac that secretes sepia, a dark fluid containing the pigment melanin.
• When a predator tries to attack, the animal ejects the ink into the water where it hangs between the animal and the predator screening a quick escape.

Class Cephalopoda - Reproduction

• Sexes are separate in cephalopods.
• Juveniles hatch directly from eggs – no free-swimming larvae.
• One arm of male is modified as an intromittent organ, the hectocotylus.
• Removes a spermatophore from mantle cavity and inserts it into female.

Class Cephalopoda

• Most octopuses creep along the seafloor in search of prey.

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Class Cephalopoda

• Squids use their siphon to fire a jet of water, which allows them to swim very quickly.

Class Cephalopoda

• One small group of shelled cephalopods the nautiluses, survives today.

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