Integrated Principles of Zoology
Eighteenth Edition
Chapter 16
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Molluscs 1
©Larry Roberts/McGraw-Hill Education
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A Shell-Collector’s Delight
Molluscs are astonishingly diverse.
Include worm-like creatures, giant squids, and animals with one, two, eight, or no shells.
Common shared features include unusual ribbon of teeth called radula for feeding; large muscular foot or tentacles for movement; and mantle for respiration and shell secretion.
Humans use shells for food, money, jewelry, and decorations around the world.
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Molluscs 2
One of the largest animal phyla with over 90,000 living species and 70,000 fossil species.
Soft body; belong to the group of lophotrochozoan protostomes.
Develop via spiral cleavage and with schizocoelous coelom.
Include chitons, tusk shells, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squids, octopuses, and nautiluses.
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Mollusc Diversity
Range in size from microscopic to 900 kg and 20 m long; 80% are under 10 cm in size.
Have various nutritional modes ranging from herbivorous grazers, predaceous carnivores, filter feeders, detritus feeders, and parasites.
Most are marine, some are terrestrial or live in freshwater; range in habitats from warm all the way to polar seas.
Can be free floating, pelagic, sessile, or burrower bottom feeder types.
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Examples of Mollusc Diversity
Figure 16.1 Molluscs: a diversity of life forms. (A) A chiton (Tonicella lineata), class Polyplacophora. (B) A marine snail (Calliostoma sp.), class Gastropoda. (C) A nudibranch (Chromodoris sp.), class Gastropoda. (D) Geoduck clam extend their large siphons, class Bivalvia. (E) An octopus (Octopus briareus), class Cephalopoda.
a: ©Kjell B. Sandved/Science Source b: ©Carver Mostardi/Alamy (c,e): ©Larry Roberts/McGraw-Hill Education d: ©Doug Wilson/Alamy
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Evolution of Molluscs
Fossil evidence indicates molluscs evolved in the sea; most have remained marine due to abundant food resources and various available habitats.
Some bivalves and gastropods moved to brackish and freshwater habitats; bivalves were unable to leave the aquatic area since they are filter feeders.
Slugs and snails successfully invaded land but are limited to moist, sheltered habitats with calcium in the soil.
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Mollusc Relationships
Figure 16.2 Cladogram showing hypothetical relationships among classes of Mollusca.
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Economics of Molluscs
Many are used as food while culturing of pearls and pearl buttons is an important industry around the world.
Other molluscs are pests such as burrowing shipworms that destroy wooden ships and wharfs while snails and slugs are common garden pests.
Some snails are intermediate hosts for parasites that affect humans and domesticated animals.
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Head-Foot
Head-foot is the area that contains the feeding, sensory, and locomotory organs.
Most have well-developed head bearing mouth and some sensory organs.
Photosensory receptors range from simple to complex eyes especially in cephalopods.
Tentacles may be present for some species.
Has radula within the mouth, which is the most unique feature of molluscs.
Posterior to mouth is the chief locomotory organ, the foot.
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General Mollusc Body Plan
Figure 16.3 Generalized mollusc.
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Radula
Unique to molluscs; found in all except bivalves and some solenogasters.
Protruding, rasping, tongue-like organ that is a ribbon-like membrane with rows of tiny teeth (up to 250,000) pointed backwards.
Complex muscles, called odontophores, move radula and its supportive cartilages in and out of the mouth while the membrane is partly rotated over the tips of the cartilages.
Radula rasps off particles of food from surfaces and serves as a conveyor belt to move particles to digestive tract.
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Radula Teeth
New rows of teeth are secreted in the posterior and replace those that wear away in the anterior end.
Pattern and number of teeth are used in the classification of molluscan species.
Some radulas are specialized to bore through hard material or for harpooning prey.
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Structure of a Radula
Figure 16.4 (A) Diagram of a gastropod head showing a radula and radula sac. (B) Radula of a snail prepared for microscopic examination.
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Foot
Usually ventral and used as attachment to substratum or for locomotion.
Numerous modifications include attachment disc of limpets, hatchet foot of clams, and siphons for jet propulsion in cephalopods.
Secreted mucus aids in adhesion or helps molluscs glide on cilia.
Snails and bivalves extend the foot hydraulically by engorgement with blood and burrow into mud or sand by enlarging the tip as an anchor.
Free-swimming forms use modified wing-like or fin-like parapodia for locomotion.
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Mantle and Mantle Cavity
Part of the visceral mass of a mollusc.
Mantle is a sheath tissue on each side of the body that protects the soft parts and forms the space called mantle cavity.
Outer surface of mantle secretes the shell in species that have shells.
Mantle cavity houses the gills or lungs that develop from the mantle and the exposed surface used in gas exchange.
Products of digestive, excretory, and reproductive systems empty into the mantle cavity.
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Mantle Cavity Function
In aquatic molluscs, continuous water flow brings in oxygen and food, flushes out wastes and reproductive materials.
Cephalopods use head and mantle cavity to create jet propulsion.
Gastropods can withdraw head and foot into the mantle cavity and hide into the shell.
Mollusc gill (ctenidium) has leaf-like filaments with cilia that propel water across the surface.
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Primitive Mollusc Gill
Figure 16.5 Primitive condition of mollusc gill (ctenidium). Black indicates ciliary current while red is blood flow.
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Shell
Secreted by and lined by the mantle.
Generally has three layers.
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Structure of the Shell
Figure 16.6 (A) Vertical section of shell and mantle of a bivalve. (B) Formation of pearl between mantle and shell as a parasite or bit of sand becomes covered with nacre.
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Respiration, Circulation, and Digestion
Gas exchange via specialized respiratory organs such as ctenidia, secondary gills, lungs, body surface, and the mantle.
Open circulatory system present with pumping heart, blood vessels, and blood sinuses; less efficient in supplying oxygen to the body compared to closed system.
Most cephalopods have a closed system with a heart, vessels, and capillaries and enable them to move actively.
Digestive tract is complex and highly specialized for various feeding habits.
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Excretion, Osmoregulation, and Senses
Most molluscs have a pair of kidneys, or metanephridia, in which inner ends open into the coelom via nephrostome.
Kidney ducts also discharge sperm and eggs.
Nervous system consists of pairs of ganglia but generally simpler than in annelids and arthropods.
In air-breathing snails, nervous system produces growth hormones and maintains osmoregulation.
Sense organs vary and may be highly specialized.
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Reproduction and Life History
Most are dioecious, some are hermaphroditic.
Egg hatches and produces a free-swimming trochophore larva.
Larva undergoes direct metamorphosis into a small juvenile, as in chitons, which is ancestral for most molluscs.
In many gastropods and bivalves, a uniquely molluscan intermediate larval stage, the veliger, is hatched with the beginnings of a foot, shell, and mantle.
Cephalopods, some freshwater bivalves, and snails have no free swimming larvae and only juveniles hatch directly from eggs.
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Trochophore Larvae
Figure 16.7 (A) Generalized trochophore larva. (B) Trochophore of a Christmas tree worm, Spirobranches spinosus (Annelida).
b: ©Dr. Thurston C. Lacalli/University of Victoria
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Veliger Larva
Figure 16.8 Veliger of a snail, Pedicularia sp.
©Kjell B. Sandved/Science Source
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Class Caudofoveata
About 120 species of worm-like, marine organisms ranging from 2 to 140 mm long.
Most burrow and orient vertically with terminal mantle cavity and gills at entrance.
Have oral shield and radula used for food selection and intake.
Feed primarily on microorganisms and detritus.
Dioecious and have one pair of gills.
Group also called Chaetodermomorpha.
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Class Solenogastres
Approximately 250 species; similar to caudofoveates, but have no radula or gills; may have secondary respiratory structures.
Foot has a midventral, narrow furrow called the pedal groove.
Do not burrow but are bottom dwellers and feed on cnidarians.
All are hermaphroditic.
Group sometimes called Neomeniomorpha.
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Solenogaster and Caudofoveate
Figure 16.9 Spicules are clearly visible on the skin of solenogasters and caudofoveates. (A) Neomeniomorpha, a solenogaster. (B) Chaetoderma elegans, a caudofoveate.
©2006, L. Kuhnz/MBARI
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Class Polyplacophora: Chitons
Generally flattened dorsoventrally with a convex dorsal surface that has seven or eight articulating limy plates.
About 1000 currently described species that range from 2 cm to 30 cm.
Prefer rocky surfaces along intertidal regions; some found at great depths.
Head and cephalic organs are reduced.
Photosensitive structures (esthetes) similar to eyes pierce the plates.
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Mossy Chiton
Figure 16.10 Mossy chiton, Mopalia muscosa.
©Randimal/Shutterstock
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Chiton Form
Chiton radula is reinforced with iron mineral called magnetite and used to scrape algae from the rocks.
Has broad muscular foot that clings on rocks but can roll up when detached.
Mantle forms girdle and extends around margin of the plates.
Gills are numerous and suspended from roof of mantle cavity along sides of foot.
Mantle and foot edge form grooves that enclosed chambers where water flows from anterior to posterior.
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Chiton Function
At low tide, chitons tightly press on rocks to seal edges and prevent water loss.
Pair of osphradia (chemoreceptive sense organs) for sampling water are in mantle groove in many species.
Blood pumped by a three-chambered heart and travels through aorta and sinuses then to the gills.
Pair of metanephridia carries wastes from pericardial cavity to exterior.
Sexes are separate with trochophore larvae changing to juveniles without a veliger stage.
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Structure of a Chiton
Figure 16.11 Anatomy of a chiton (class Polyplacophora). (A) Longitudinal section. (B) Transverse section. (C) External ventral view.
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Class Monoplacophora
Previously considered extinct; living specimen discovered in 1952 and about 25 extant species now known.
Small molluscs with radula, a rounded shell, and creeping foot that resemble limpets.
Some organs are repeated: 3 to 6 pairs of gills, 2 pairs of heart atria, 3 to 7 pairs of metanephridia, with 1 to 2 pairs of gonads and 10 pairs of pedal nerves.
Studies suggest that Monoplacophorans and Polyplacophorans are sister phyla, as each has serially repeated structures.
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Structure of Monoplacophora
Figure 16.12 Neopilina, class Monoplacophora. (A) Ventral view. (B) Dorsal view.
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Class Gastropoda
Most diverse class with over 70,000 living and more than 15,000 fossil species.
Range in forms such as marine snails, limpets, slugs, whelks, conches, periwinkles, sea slugs, sea hares, and sea butterflies, to air-breathing terrestrial snails and slugs.
Typically slow, sedentary animals due to heavy shells used as chief defense.
Some snails are specialized for climbing, swimming, or burrowing.
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Gastropod Shells
One-piece univalve, coiled or uncoiled.
Apex is smallest and oldest part of whorl.
Whorls get larger as they spiral around central axis (columella).
Shells may be dextral (right sided) or sinistral (left sided) in coiling formation.
Many snails have operculum covering shell aperture (entry).
Giant marine gastropods have shells up to 60 cm and some fossil forms are 2 m long.
Occupy all kinds of habitats that range from great altitudes to even polar regions and the deep sea.
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Structure of Gastropod Shell
Sinistral, or left-handed shell.
Dextral, or right-handed shell
Figure 16.13 Shell of the whelk Busycon sp.
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Gastropod Features
Terrestrial gastropods are restricted by soil mineral content, temperature, dryness, and acidity but are still widespread in all areas.
Aside from shells, snails also release distasteful or toxic secretions with some deploying stinging cells of cnidarian prey or forming poison harpoons to capture fish and other faster swimming organisms.
Certain snails, such as Strombus sp., have a modified operculum with a sharp spike to give an active blow to possible predators.
Some snails serve as intermediate hosts to many parasites and are often harmed by larval stages during infestation.
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Ontogenetic Torsion
Torsion is developmental process that changes the relative position of the shell, digestive tract and anus, nerves that lie on both sides of the digestive tract, and the mantle cavity containing the gills.
Contraction of asymmetrical foot retractor muscle pulls shell and viscera 90 degrees counterclockwise relative to head region.
Moves anus from posterior to the right side of the body.
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Process of Torsion
Initial movements of shell rotates it between 90 and 180 degrees into a permanent position.
Mantle cavity develops on the right side of the body near the anus, but is initially separate from it.
Anus and mantle cavity usually move further to the right and the mantle cavity is remodeled to encompass the anus.
Digestive tract moves both laterally and dorsally so that anus lies above head within mantle cavity.
After torsion, anus and mantle cavity open above mouth and head.
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Gastropod Torsion
Figure 16.14 Ontogenetic torsion in a gastropod veliger larva.
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Result of Torsion
Certain viscera (left gill, kidney, and heart) now on the right side and vice versa; nerve cords form a figure eight structure.
Mantle cavity now has more space in which the head can be withdrawn into the shell with the muscular foot and operculum, forming a barrier.
Varying degrees of detorsion has occurred in opistobranchs and pulmonates with anus opening to the right side or the posterior region.
The arrangement resulting from torsion creates fouling where wastes are washed back over the gills.
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Coiling
Coiling is spiral winding of shell and visceral mass.
Occurs at same larval stage as torsion but had a separate, earlier evolutionary origin.
All living gastropods descended from coiled, torted ancestors with some losing this feature.
Early planospiral shell had all whorls in single plane; not compact as each whorl completely outside the preceding one.
Conispiral shape provides more compactness with each whorl to the side of the previous one.
Shifting the shell upward and back helped balance uneven weight distribution.
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Consequences of Shell Weight
Weight and bulk of shell, especially the largest whorl, has now pressed on the mantle cavity and can interfere with organ function such that the gill, heart atrium, and kidney of right side are lost in most living species.
Loss of the right gill provides one solution to the problem of fouling and wastes expel to the right side from the anus and nephridiopore.
Bilateral symmetry is achieved with this body arrangement for some species but others have reverted back to planospiral shell forms.
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Shell Evolution
Figure 16.15 Evolution of the shell in gastropods.
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Gastropod Feeding Habits
Adaptation of the radula provides much variation in gastropod feeding habits.
Snails detect chemical cues from metabolic wastes of potential prey.
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Red Abalone and Moon Snail
Figure 16.16 (A) Red abalone, Haliotis rufescens. (B) Moon snail, Polinices lewisii.
a: ©Dr. Dwayne Meadows, NOAA/NMFS/OPR b: ©Burt Jones & Maurine Shimlock/NHPA
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Cone Snails
Species of Conus snails feed on fish, worms, and molluscs using a lethal sting to secure prey.
Several species of Conus are lethal to humans and serve as valuable tools in research on cell receptors and ion channels of nerve cells.
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Cone Snail Capturing Prey
Figure 16.17 Conus sp. extends its long, worm-like proboscis. (A) Stings the fish in the mouth and kills it. (B) The snail engulfs the fish with its distensible stomach.
a: ©Franco Banfi/Science Source b: ©Robert F. Sisson/Contributor/Getty Images
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Other Feeding Strategies
Some gastropods, such as the queen conch, feed on organic deposits.
Others are ciliary feeders, using gill cilia to collect debris as a mucus ball that is moved to the mouth.
Sea butterflies secrete a mucus net to catch small plankton and then pull net into their mouth.
Digestion in ciliary feeders is intracellular.
Others use radula and gizzard to macerate food and digestion is extracellular within the lumen of the stomach and digestive glands.
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Gastropod Respiration
Respiration performed by a ctenidium in mantle cavity while others have no gills and use the mantle and skin.
Some prosobranchs, after losing one gill, lost half of remaining gill such that attachment to wall of mantle cavity provided the most respiratory efficiency.
Pulmonates lack gills but have highly vascular area in the mantle that serves as a lung; it opens to outside by small opening, the pneumostome.
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Gill Evolution
Figure 16.18 Evolution
of gills in gastropods.
(A) Primitive condition. (B) Condition after one gill had been lost.
(C) Derived condition found in most marine gastropods.
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Gastropod Excretory and Nervous Systems
Most gastropods have a single kidney called a nephridium and well-developed circulatory and nervous systems.
Sense organs include eyes, statocysts, tactile organs, and chemoreceptors.
Eyes vary from simple cup-like indentations on the skin that hold pigmented photoreceptors to complex eyes with a lens and cornea.
Sensory osphradium at the base of the incurrent siphon is chemosensory for some species or mechanoreceptive for other forms.
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Snail Anatomy
Figure 16.19 Anatomy of a pulmonate snail.
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Gastropod Reproduction
Gastropods have monoecious and dioecious species that have various courtship ceremonies.
Primitive reproductive feature is external fertilization forming free swimming trochophores; most have internal fertilization.
Eggs laid singly or in clusters, sometimes in tough capsules.
Young usually hatch as veliger larvae; sometimes as snails.
Some, including most freshwater snails, are ovoviviparous and brood eggs and young in the oviduct.
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Gastropod Eggs
Figure 16.20 Eggs of marine gastropods. (A) Kellet’s whelk, Kellettia keletia, lays egg cases resembling grains of wheat. (B) Egg ribbon of a nudibranch.
a: ©Juniors Bildarchiv GmbH/Alamy b: ©Fauzan Abubakar/Shutterstock
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Major Groups of Gastropods
Traditional classification has recognized three subclasses of Gastropoda:
The number of subclasses in Gastropoda have been in considerable controversy for many years but are still used in this textbook for convenience sake.
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Prosobranchs
Includes most marine snails, some freshwater and terrestrial gastropods.
Mantle cavity is anterior due to torsion.
Gills are in front of heart; water enters the left side and exits from the right side.
Long siphons may separate incurrent and excurrent flow.
Have one pair of tentacles, separate sexes, and usually an operculum is present.
Range in size from small periwinkles to large horse conch that grow up to 60 cm in length (the largest Atlantic gastropods).
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Examples of Prosobranchs
Figure 16.21 (A) The limpet, Patella, has a single flattened cap-like shell. (B) Flamingo tongues, Cyphoma gibbosum, are showy inhabitants of Caribbean coral reefs.
a: ©Paul Kay/Getty Images b:©Larry Roberts/McGraw-Hill Education
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Opisthobranchs
Most are marine, shallow-water and often hide under stones and seaweed; some are pelagic.
Have partial to complete detorsion with anus and gill(s) displaced to right side or rear.
Two pairs of tentacles with one pair modified to increase chemo-absorption called rhinophores.
Shell is reduced or absent and all are monoecious.
Sea hare (Aplysia) have large ear-like anterior tentacles and a vestigial shell.
Foot of pteropods (sea butterflies) is modified into fins for swimming in pelagic areas.
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Sea Hare
Figure 16.22 (A) A sea hare, Aplysia dactylomela, crawls and swims across a coral reef, assisted by large, wing-like parapodia. (B) When attacked, sea hares squirt a copious protective secretion derived from their red algal food source.
(a,b): ©Cleveland P. Hickman, Jr.
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Nudibranch
©Larry Roberts/McGraw-Hill Education
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Pulmonates
Ancestral ctenidia have been lost and the vascularized mantle wall is now a lung.
Anus and nephridiopore open near the pneumostome and waste is forcibly expelled.
Show some detorsion.
Generally monoecious.
Aquatic species have one pair of nonretractile tentacles with eyes at the base, land species have two pair of tentacles with the posterior pair having eyes.
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Land Snail and Slug
Figure 16.24 (A) Pulmonate land
snail. Note two pairs of tentacles; the second, larger pair bears the eyes. (B) Banana slug, Ariolimax columbianus.
a: ©Larry Roberts/McGraw-Hill Education b: ©Cleveland P. Hickman, Jr.
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Class Bivalvia
Also called Pelecypoda or “hatchet-foot.”
Range in size from 1 to 2 mm in length to the giant South Pacific clams of more than 1 m and 225 kg.
Most are sedentary filter feeders and depend on currents produced by cilia.
Bivalves lack a head, radula, or other aspects of cephalization.
Most are marine but many live in brackish water and also in streams, ponds, and lakes.
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Examples of Bivalves
Figure 16.25 Bivalve molluscs. (A) Mussels, Mytilus edulis, occur in northern oceans around the world. (B) Scallops, Chlamys opercularis, swim to escape attack by starfish, Asterias rubens.
a: ©Andrew J. Martinez/Science Source b: ©D.P. Wilson/FLPA/Science Source
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Bivalve Shell
Bivalves are laterally compressed with two shells or valves that are held together by a hinge ligament.
Valves are drawn together by strong adductor muscles.
Umbo is the oldest part of the shell with growth occurring outward in rings.
Pearls are produced when an irritant is lodged between the shell and mantle.
Layers of nacre are secreted around the foreign material.
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Structure of a Bivalve
Figure 16.26 Tagelus plebius, stubby razor clam (class Bivalvia). (A) External view of left valve. (B) Inside of right shell. (C) and (D) Sections showing function of adductor muscles and hinge ligament.
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Bivalve Body and Mantle
Visceral mass is suspended from the dorsal midline.
Muscular foot is attached anteroventrally.
Ctenidia hang down on each side, each covered by a fold of the mantle.
Posterior edges of the mantle folds form excurrent and incurrent openings.
In burrowing clams, mantle forms long siphons to reach the water above the substrate.
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Bivalve Visceral Mass
Figure 16.27 Evolution of bivalve ctenidia and section through a bivalve shell and body, showing relative positions of visceral mass and foot.
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Bivalve Siphons
Figure 16.28 Adaptations of siphons in bivalves. (A) In the pholad clam from Monterey, California, incurrent and excurrent siphons are clearly visible above the sediment surface. (B) to (D) In many marine forms, the mantle is drawn out into long siphons.
(a): ©Steve Trewhella/FLPA/Science Source
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Bivalve Locomotion
Foot is extended out from between the valves when blood is pumped into the foot.
As foot swells, it anchors the bivalve in the mud.
When longitudinal muscles shorten, the foot pulls the clam forward.
Scallops and file shells swim by clapping their valves to create a jet propulsion.
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Bivalve Gills
Both mantle and gills perform gaseous exchange.
Gills are also used for filter-feeding and were derived from primitive ctenidia by lengthening of the filaments to each side.
Filaments fused to form plate-like lamellae with vertical water tubes inside.
Water enters incurrent siphon propelled by cilia then passes into water tubes through pores between the filaments.
Water proceeds dorsally to the suprabranchial chamber and exits through the excurrent siphon.
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Bivalve Respiration and Circulation
Figure 16.30 Section through heart region of a freshwater clam to show relation of circulatory and respiratory systems.
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Bivalve Feeding
Most bivalves are filter feeders; respiratory currents bring both oxygen and organic materials to the gills.
Other feeding strategies.
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Freshwater Clam
Figure 16.31
(A) Feeding mechanism of freshwater clam. (B) Clam anatomy.
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Shipworm
Figure 16.32 (A) Shipworms are bivalves that burrow in wood, and are nicknamed “termites of the sea.” (B) The two small, anterior valves, seen at right, are used as rasping organs to extend the burrow.
(a,b): ©Larry Roberts/McGraw-Hill Education
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Giant Clam
Figure 16.33 Giant Clam (Tridacna gigas) lies buried in coral rock with greatly enlarged siphonal area visible.
©Larry Roberts/McGraw-Hill Education
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Bivalve Digestion
Stomach is folded into ciliary tracts for sorting particles.
Style sac secretes a crystalline style which is kept whirling by cilia in style sac.
Rotating style helps free digestive enzymes and roll up a mucous food mass.
Dislodged particles are directed to a digestive gland or are engulfed by amebocytes with further digestion being intracellular.
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Clam Stomach and Style
Figure 16.34 Stomach and crystalline style of ciliary-feeding clam. (A) External view of stomach and style sac. (B) Transverse section showing direction of food movements. (C) Sorting action of cilia.
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Bivalve Circulation and Excretion
Three-chambered heart has two atria and one ventricle within the pericardial cavity.
Pair of U-shaped kidneys called nephridial tubules lie ventral and posterior to heart.
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Bivalve Nervous System
Nervous system has three pairs of widely separated ganglia connected together.
Sense organs are poorly developed.
Scallops have a row of small, blue mantle eyes that have a cornea, lens, retina, and pigmented layer.
Tentacles on the margin of mantle may have tactile and chemoreceptor cells.
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Scallop
Figure 16.29 The surface-dwelling scallop Aequipecten irradians has developed sensory organs along its mantle edges like tentacles and a series of blue eyes.
©Larry Roberts/McGraw-Hill Education
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Bivalve Reproduction
Sexes usually separate.
Gametes discharged in suprabranchial chamber are carried out in excurrent flow.
Fertilization usually external with embryos developing as trochophore, veliger, and spat larval stages.
Freshwater clams have internal fertilization with sperm entering via the incurrent siphon to fertilize eggs in the water tubes of the gills and developing into a bivalved glochidium larvae.
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Oyster Life Cycle
Figure 16.35 Life cycle of oysters.
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Glochidium Larvae
Glochidium larvae are specialized veligers that attach to gills of passing fish, where they live briefly as parasites to complete their development.
Some species release glochidia into water, others attempt to lure potential hosts to brooding female.
After encysting on a suitable host for development, the juveniles detach and sink to the substrate to begin independent life on the streambed.
“Hitchhiking” larvae have helped distribute the species even with limited locomotion.
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Glochidium Larva and Mantle Lure
Figure 16.36 (A) Glochidium, or larval form, for some freshwater clams. (B) The mantle edge of this female pocketbook mussel (Lampsilis ovata) mimics a small minnow to attract potential host.
v©Todd J. Morris, PhD
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Boring Bivalves
Burrowing in sand has led some species to evolve a mechanism for boring into harder surfaces like wood and rock.
Shipworms are destructive to ships and wharfs since they use a pair of globular valves with microscopic teeth for drilling.
Some clams bore into rock.
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Class Scaphopoda
Commonly known as tusk or tooth shells.
Mostly benthic marine species found in subtidal zone extending to 6000 m depth.
About 900 living species that range from 2.5 to 5 cm long with separate sexes.
Generally slender body covered with a mantle and a tubular shell that is open at both ends.
Unique body plan with mantle wrapped around the viscera and fused to form a tube.
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Scaphopoda Features
Foot protrudes from larger end of shell to burrow into mud or sand.
Movement of foot and ciliary action circulates water through mantle cavity.
Gills are absent and gaseous exchange occurs via the mantle.
Food detritus and protozoa are caught on cilia on foot or mucus-covered knobs of tentacles called captacula.
Radula carries food to a crushing gizzard.
Head lacks eyes, osphradia, and other sensory features.
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Tusk Shell
Figure 16.37 The tusk shell, Dentalium (class Scaphopoda). (A) It burrows into soft mud or sand and feeds by means of its prehensile tentacles (captacula). (B) Internal anatomy of Dentalium.
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Class Cephalopoda
All are marine predators such as squids, octopuses, nautiluses, devilfish, and cuttlefish.
Foot is merged with the head region and modified for expelling water from mantle cavity.
Range from 2 cm to 20 m including the giant squid, which is the largest invertebrate.
Fossil record goes back to the Cambrian.
Earliest shells were straight cones, others were coiled or curved.
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Cephalopod Diversity
Nautilus is the remaining survivor of nautiloids, which exhibit a highly coiled shell.
Octopuses and squids apparently evolved from early straight-shelled ancestors.
Ammonoids are extinct but had elaborate shells.
Cephalopods are mostly marine and sensitive to salinity.
Octopuses mostly intertidal, hiding in rocks and crevices, while squids are deep-sea animals.
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Nautiloid and Ammonoid
Figure 16.38 Nautilus sp., a cephalopod. (A) Live Nautilus sp., feeding on a fish. (B) Longitudinal section, showing gas-filled chambers of shell. (C) Longitudinal section through shell of an ammonoid.
(a): ©Image Source/Alamy
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Cephalopod Shell
Nautiloid and ammonoid shells have gas chambers allowing them to remain buoyant.
Cuttlefish shell is small, curved, and entirely enclosed by the mantle.
Squid shell is a thin proteinaceous strip called the pen, enclosed by mantle.
Octopus has completely lost the shell.
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Cuttlefish
Figure 16.39 Cuttlefish, Sepia latimanus, has an internal shell familiar to keepers of caged birds as “cuttlebone.”
©Larry Roberts/McGraw-Hill Education
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Cephalopod Locomotion
Cephalopods swim by forcefully expelling water through a mobile ventral funnel or siphon.
Body of squids and cuttlefishes is streamlined with lateral fins as stabilizers.
Nautilus swims mainly at night using gas chambers to maintain position.
Octopuses have globular body and no fins; normally crawl over rocks and corals using tentacles and suction cups.
Octopuses can also swim backward by spurting jets of water through funnel.
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Structure of Squid
Figure 16.40 (A) Lateral view of squid anatomy, with the left half of the mantle removed. (B) Loligo vulgaris, from the Mediterranean Sea.
(b): ©Borut Furlan/Getty Images
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Cephalopod Circulation and Respiration
Cephalopods have active lifestyles and well-developed internal anatomy.
Except for nautiloids, cephalopods have only one pair of gills with no cilia around the gills.
With higher oxygen demands, cephalopods have a muscular pumping system to keep water flowing through the mantle cavity.
Closed circulatory system has a network of vessels conducting blood through gill filaments.
Accessory or branchial hearts at the base of each gill increase pressure to blood going through gill capillaries.
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Cephalopod Nervous System
Cephalopod nervous and sensory systems are more elaborate than other molluscs.
Cephalopod brain is the largest of any invertebrate with several lobes and millions of nerve cells; while squids have the largest nerve fibers in the animal kingdom.
Sense organs are well-developed with complex eyes complete with cornea, lens, and retina that have high visual acuity but are generally color-blind.
Cephalopods lack a sense of hearing but have tactile and chemoreceptor cells in their arms.
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Cuttlefish Eye
Figure 16.41 Eye of a cuttlefish (Sepia). The structure of
cephalopod eyes is very similar to that of eyes of vertebrates.
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Cephalopod Intelligence
Cephalopods have been taught to differentiate shapes and can remember such differences for quite some time.
Researchers have shown that the octopus is capable of observational learning.
Octopods have been seen to discriminate between surface textures by using their tentacles to feel.
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Cephalopod Communication
Little known of social behavior in nautiloids and deep-water cephalopods.
Most nearshore species use chemical and visual signals to communicate.
Body movements ranging from small motions to exaggerated spreading, curling, and other arm actions along with color change are used.
Ink sac containing an ink gland used as protective device.
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Cephalopod Color Changes
Chromatophores are skin cells containing pigment granules.
Many deep-water species have elaborate luminescent organs.
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Cephalopod Reproduction
Sexes are separate with males showing various color displays to ward off other males.
In male seminal vesicle, spermatozoa are packaged in spermatophores and stored in a sac that opens into the mantle cavity.
One arm of male is modified as an intromittent organ, a hectocotylus, which is used to remove a spermatophore from the male mantle cavity and insert it into the female oviduct.
Fertilized eggs leave oviduct and are attached to stones and other substrates; some octopus species tend to their eggs.
Juveniles hatch from eggs; no free-swimming larvae exist.
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Cephalopod Copulation
Figure 16.42 Copulation in cephalopods. (A) Mating cuttlefishes. (B) Male octopus uses modified arm to deposit spermatophores in female mantle cavity.
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Nautiloidea
Nautiloidea have two pairs of gills.
Nautilus is the only surviving genus in Nautiloidea that populated the Paleozoic and Mesozoic seas.
There are five living species.
Head has 60 to 90 tentacles that can extend from the opening of the shell.
Tentacles lack suckers but have adhesive secretions; used for searching, sensing, and grasping food.
Beneath head is the funnel, mantle cavity, and visceral mass sheltered by a shell.
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Ammonoidea
Ammonoidea were prevalent during the Mesozoic but were all extinct at end of the Cretaceous.
Chambered shells resembled those of the nautiloids but septa were more complex and frilled.
Reason for extinction is unknown, as new evidence indicate they died out before the asteroid bombardment at the end of the Cretaceous.
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Coleoidea
Coleoidea have one pair of gills.
Orders of Coleoidea include all living cephalopods except Nautilus.
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Mollusc Phylogeny
First mollusc probably arose in Pre-Cambrian times with fossils dating back to early Cambrian period.
Molluscs are protostomes, allied with annelids in the lophotrochozoa.
Opinions differ about the exact nature of the relationship among lophotrochozoans but annelids and molluscs are coelomate protostomes, though annelids are metameric.
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Relationships of Mollusc Groups
One study supports grouping Monoplacophora and Polyplacophora into clade Serialia, nested within a clade of unsegmented molluscs.
Other studies support grouping caudofoveates, solenogasters, and chitons into clade Aculifera.
Some studies place cephalopods as sister taxon to gastropods but new work places them with monoplacophorans.
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Adaptive Diversification
Diversity of molluscs related to their adaptation to different habitats and to a wide variety of feeding methods.
The shell and mantle has had a wide range of evolutionary adaptations that make molluscs a very successful group today.
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Evolution and Abundance of Molluscs
Figure 16.43 Classes of Mollusca, showing their derivations and relative abundance.
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Mollusc Relationships - Text Alternative
Molluscs are united by shared features such as a radula and mantle. Phylum Mollusca splits into major clades, Aculifera and Conchifera. Clade Aculifera includes classes Caudofoveata, Solenogastres, and Polyplacophora. The Conchifera includes 5 classes - Monoplacophora, Gastropoda, Bivalvia, Scaphopoda, and Cephalopoda.
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Trochophore Larvae - Text Alternative
A trochophore larva is roughly a three dimensional diamond. There are apical tufts of cilia, and a band of cilia around the middle, or equator, of the larva. The mouth is located on one side, near the band of cilia while the anus is near the lower apical tuft of cilia.
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Gill Evolution - Text Alternative
Cross sections of a gastropod, showing the change in gill structure from the primitive to the modern condition. The primative condition is two gills, one on each side of the mantle cavity, each attached to the mantle wall. Water would enter the cavity, flow over the gills, and exit through a dorsal hole. After one gill had been lost, the mantle cavity shifted slightly, enlarging around the remaining gill. Water entered the cavity, flowed over the gill, and exited at the other side of the cavity where the other gill was previously located. The derived condition found in most modern molluscs is to have one gill with an axis attached to the mantle wall. Water enters the cavity near the gill, flows over it, and exits out the opposite side.
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Nautiloid and Ammonoid - Text Alternative
The nautilus has a coiled shell divided into chambers by septa. The animal lives in the last and largest chamber, with tentacles extending out of the shell. A cord of tissue, the siphuncle, runs down the center of the chambers to connect them. The ammonoid shell is similar, but the siphuncle canal is on the outer edge of the spiral, not in the center of the chambers. The septa of ammonoids are wavy rather than being smooth like the nautiloid.
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Evolution and Abundance of Molluscs - Text Alternative
Molluscs evolved in the Precambrian and diversified into at least 6 groups. The Monoplacophora briefly increased in abundance early in the Paleozoic, but then shrunk in diversity, maintaining that lower diversity level until the present. The gastropods and cephalopods spit from each other early in the Paleozoic; each increased in diversity. The gastropods are the most abundant group of molluscs today while cephalopd diversity declined in the Mesozoic. The bivalves and scaphopods split from each other early in the Paleozoic. The scaphopods have had limited diversity throughout their existence while the bivalves increased dramatically in abundance. Polyplacophorans increased slightly in abundance in the Paleozoic and maintained that to the present. Solenogasters and caudofoveates have exhibited low abundance since they evolved in the Precambrian.
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