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Flashcards in Lecture 4 RH Deck (97)

What is the importance of molluscs?

Highly diverse and successful phylum

Economically important for food, they act as pests, popular culture (eg kraken), ecologically important, vectors/intermediate hosts for parasites


What are the classes of molluscs?


Polyplacophora - chitons gastropods




What is the body plan of a mollusc like?

Radula (head) and foot

Visceral mass within mantle and mantle cavity surrounded by a shell and contains internal organs


What body systems to molluscs have?






What is the odontophore and what is it used for?

A feeding organ which uses radula like teeth which scrape and collect food . New teeth are constantly produced at the posterior end.


Where is the foot usually located?



What does the foot do?

Attaches the mollusc to the substratum and allows locomotion

Secretes mucous to allow in adhesion or gliding on cilia


What are some modifications that the foot has had throughout evolution?

Attachment of disc of limpets

Hatchet foot of clams

Siphon jet of squids


How do snails and bivalves move using their foot?

they extend it hydraulically by engorgement with blood


How do burrowers use the foot?

They extend the tip and use it as an anchor to drag themselves forward


How do free-swimming molluscs take advantage of the foot for swimming?

Foot is modified into wing or fin-like swimming agents


Where is the visceral mass located?

Within the mantle and mantle cavity


What is the mantle?

A sheath of skin on each side of the body which secretes the shell when present.


What is the function of the mantle?

Produces shell

Exposed part is important for gas exchange

Houses gills or lungs that develop from the mantle

In aquatic molluscs it ensures a continuous flow of water to bring oxygen and food in and flush out wastes

Products of digestive, excretory, and reproductive systems empty into the mantle

Cephalopods use the head and mantle for jet propulsion


What is the periostracum?

Protein layer (conchiolin) which is a horny outer shell layer


What is the prismatic layer?

The middle layer with closely packed prisms of calcium carbonate


What is the nacreous layer?

The inner layer next to the mantle which is laid down in thin layers


What are the layers of the shell of molluscs?

Periostracum (outer horny layer)

Prismatic layer (calcium carbonate)

Nacreous (thin layers)


What does periostracum do in freshwater molluscs?

Thick periostracum of freshwater molluscs protects from acid from leaf decay


Where are mollusc gills located?

housed by mantle cavity created by overhanging mantle and shell


What are ctenidia?

Flattened filaments extending alternately from supporting axis.

These are gills


Are gills bi or monopectinate?

Some are mono others are bipectinate


How does water move past the gills of a mollsuc?

Leaf-like filaments (cilia) which propel water across the surface countercurrent to absorb oxygen efficiently


What are the internal structures of the visceral mass?

Pumping heart

Blood vessels

Blood sinuses (haemocoel)


What is the name of the molluscs' kidneys?



Do molluscs have open or closed circulatory systems?

Cephalopods have closed circulatory systems


What do mollusc nervous systems look like?

pairs of ganglia and nerve cords. One pair of nerve cords innervates the foot (pedal) and another pair (visceral) innervates the visceral mass


What kind of sensory organs do molluscs have?

Tentacles, eyes, statocysts (balance), osphradia (patches of sensory epithelium in a mantle cavity which function as chemoreceptors)


What kind of digestive organs do molluscs have?

primitive stomach adapted for processing of fine food particles.

A rotating mucous mass (protostyle)

particles are separated over a sorting region and fine particles are sent up the ducts of the surrounding digestive glands where intracellular digestion occurs.


How do molluscs generally reproduce?

Dioecious; pair of gonads in visceral mass near pericardium

Fertilization is external and gametes are secreted from the metanephridia

larva are planktonic

cleavage is spiral


What are the features of development in molluscs?

Cleavage is spiral

A trochophore is the first larval stage

Veliger is the second

*trochophore unites annelids, turbellarians, nemertines, phoronids, etc in a taxon called trochozoa


What are the features of class caudofoceata and solenogastres?

Strange, wormlike molluscs found in oceans of the world to depths of 200 - 7000 meters

Small (less than 5mm length)

Head poorly developed and lack shell mantle covered by a cuticle embedded with calcareous scales or spicules

Creeping species feed on cnidarians and burrowers on small organisms and deposited material


How are caudofoceates and solengastres separated?

caudofoveates are dioecious

Solenogastres are hermaphrodites


Where were the first monoplacophorans seen and when?

First found in 1952; since then 11 species have been found and 3 genera


What are the features of monoplacophorans?

Most like generalized molluscan body plan

Repetition of both external and internal structures such as gills, retractor muscles, auricles, and kidneys

Only found in deep water (175m and below)


What does evidence show about the length of time monoplacophorans have existed for?

All deep-water specimens date back to the cambrian period


What do monoplacophorans resemble?



What are the features of polyplacophora?

Many plates on their shells

Peripheral area contains the girdle which is stiff and thick and extends beyond the lateral margin of plates.

Polyplacophora contains chitons which live on hard surfaces especially in intertidal zones

Distinct head with no eyes

size ranges from 3mm to 40cm


What do polyplacophora use for gripping?

Foot and girldle


What do the shells of polyplacophora do?

Provide protection


How big are typical polyplacophoras (chitons)?

3mm to 40cm


What do polyplacophoras eat?

most chitons are what is known as microphagous feeders.

They feed on fine algae and other organisms they scrape with their radula from rocks and shells


How many teeth do polyplacophora have on their radula?

It is very long with 17 teeth in each transverse row


Where are the gills found in polyplacophora?

between the mantle and the foot in a groove called the pallial groove


Are chitons mono or dioecious?

dioecious with single medial gonad


Where does fertilization occur in chitons?

Externally in the sea or within female mantle cavity


Where are eggs located in chitons?

Within spiny envelope and are usually shed into the sea singly or in strings


Do polyplacophora have veliger larvae?



Do polyplacophora have trochophore larvae?

Yes except in forms that brood eggs


What are features of class scaphopoda?

Burrowing marine molluscs called tusk or tooth shells.

Possess cylindrical, tusk-shaped shells open at each end

Not very successful

burrow in soft bottoms with larger anterior end downward and small posterior end upward

Feed on detritus and protozoa collected by tentacles covered in mucus


What are the features of gastropods?

Largest class of molluscs

Most successful molluscan class

Marine species adapted to life on all bottom types

Torsion and shell spiral causes right gill, auricle, and nephridium to be reduced or lost


What are the changes that occured to produce gastropods from generalized molluscs?

Development of head

Dorso-ventral elongation of the body

Converstion of the shell

Torsion or twisting of the body 180 degrees counter clockwise


What is torsion?

Viscera rotate during embryonic development


How are the head and foot retracted into the shell in gastropods?

Via retractor muscles (columella muscle)


What is the horny disk in the larvae of opisthobranchs and pulmonates called? What is its function?

The operculum; this operculum acts with the foot as a hinged lid which closes and seals the aperture


What are the subclasses of gastropods?

Prosobranchia which respire by gills in which torsion is evident (~18000 species)

Opisthobranchia which display detorsion and reduced mantle cavity. Many species are secondarily bilaterally symmetrical

Pulmonata include land snails and freshwater snails + slugs; some display detorsion, gills gone, mantle cavity is a lung.


What are the features of order archaeogastropoda?

Most basal form of living gastropods

Shell notches or perforations that permit exit of ventilating current and waste products

Largely restricted to firm substrata


What subclass is archaeogastropod a part of?



What are some examples of archaeogatropods?

slit snails, abalone, and keyhole limpets


What are the features of subclass prosobranchia?

Single left gill of higher prosobranchs is monopectinate rather than bipectinate

Not confined to hard surfaces. Many species live on soft bottoms

Freshwater and terrestrial prosobranchs are operculate


What feeding features do prosobranchs have?

Macrophagous stomachs (simple sac-like stomach)

Feeding style based on radula design

Proboscis in many carnivorous species which shoots like darts with neurotoxins


What is the mode of reproduction in prosobranchs?

Copulation and internal fertilization


What is the way development happens in gastropods?

Eggs hatch into veligers which is called direct development


What are the features of opisthobranchs?

Marine gastropods with 90 degrees of torsion (detorted)

Gill and mantle cavity are on the right side

Operculum has been lost except in single primitive family

Primitively there is a shell and gill but many opisthobranchs have reduction or loss of shell, mantle cavity and gill


What are the bilaterally symmetrical, slug-like forms of gastropods called?



What do nudibranchs represent?

Extreme reduction in shell and mantle cavity


What is the mode of reproduction in opisthobranchs?

They are hermaphrodites which copulate and fertilize internally

*they are known as protandric hermaphrodites


What stage of larvae hatch from eggs in opisthobranchs?

Veliger larvae


What are the features of pulmonates?

90 degrees of torsion (detorted) and may have evolved from mesogastropod ancestors

lung from mantle cavity replaces gills for gas exchange


Where do pulmonates typically live?

Lower pulmonates inhabit fresh water but use lung or secondary gills


How do higher pulmonates differ from lower pulmonates?

Higher pulmonates have 2 pairs of tentacles with eyes mounted on top of posterior pair.

Array of physiological and behavioral adaptations for life on land.


What is the diet of pulmonates like?

Herbivorous and carnivorous


How do pulmonates reproduce?

Hermaphrodites that exchange sperm.

Eggs are deposited within envelopes and development is direct, except in the few primitive marine species


What are the 3 groups of bivalves?

Protobranchs (3 orders of deep water clams)

Lemellibranchs (Majority of bivalves)

Septibranchs (lamellibranchs with additional septa)


What are the general characteristics of bivalves?

Bivalve shell and reduced head

Contain incurrent and excurrent siphons for water movement

Foot is laterally compressed

Adaptations for burrowing in soft substrata

Absent radula

Gill collects food instead of radula

Dioecious (exit via nephridia in protobranchs and gonoducts in other bivalves)

Parasitic glochidia present in freshwater clams


What kind of gills do protobranch bivalves have?

Bipectinate gills which are primitive


How do protobranchs feed?

They are selective deposit feeders using a pair of palpal tentacles.

Stomach contains a protostyle and sorting region.

Inner surface are ridged and ciliated; light particles are carried by crest cilia to the mouth, whereas heavy particles are carried by groove vilia to palp margins and ejected into mantle


What is the method of feeding that lamellibranchs use?

Filter feeding using gills and cilia provide vertical transport of food.


What are the lamellibranch adaptations?

Lengthened and folded gills for greater filtering surface and formation of food grooves for horizontal transport.

Water in inhalant chamber circulates between ridges and enters water tubes through numerous ostia in lamellae. Water flows out of exhalant opening

Stomach has retained primitive features associated with diet of fine particles. Protostyle has become a crystalline style which liberates enzymes.

Digestive diverticula permit intracellular digestion of protein to allow extracellular digestion of starches and lipids in stomach.


What is the connection between folded gill filaments like in primitive lamellibranchs?

Primitive lamellibranchs (fillibranchs) connections between these folds are not substantial


What is the connection between folded gill filaments like in eulamellibranchs?

Filaments are fused by a variety of junctions


How did the development of filter feeding effect the evolution of lamellibranchs?

It drove the evolution due to no longer being forced to rely on deposit material

Many lamellibranchs have remained in soft bottoms for which varying degrees of mantle fusion and siphons are important adaptations


What are the adaptations of attached surface dwellers?

They live on hard substrata and are anchored via byssal threads or cementation of one valve to substratum. Examples of these are oysters.

Foot is reduced or lost and anterior adductor muscle is common in attached bivalves


What are adaptations of unattached surface dwellers?

They live free and unattached on the surface. Some of these creatures are capable of escape swimming (eg scallops)

They swim by clapping the valves


What are the adaptations of boring bivalves?

Ability to drill into hard substrata mechanically using anterior margins as cutting tools.

Shipworms are most specialised wood borers. (they are destructive to man-made wood structures)


How do bivalves typically reproduce?

Externally via released gametes through gonoducts (nephridia in protobranchs)


Are bivalves mono or dio ecious?



What kind of existence is the design of cephalopods designed for?

Pelagic (open sea dwelling), and raptorial (predatory)


How do cephalopods swim?

Create jets of water from mantle cavity through funnel

In nautilus, funnel contracts and body retracts.

In other cephalopods, mantle wall contracts forcing water through the funnel.


How is buoyancy provided to nautilus, sepia and spirula?

Gas-filled chambers in their shells provide buoyancy. Volume is regulated via siphuncle.


What are tactics cephalopods use to escape enemies?

ink production

chromatophores in skin produce colour change


How do octopods move around?

Use secondary crawling and have a benthic existence.

Jet propulsion is only necessary for escape swimming and intermittent swimming


How do cephalopods reproduce?

Cephalopods are dioecious. A male plucks its spermatophore from its mantle and inserts it into female mantle cavity via an arm called the hectocotylus.


How do cephalopods develop?

Egg development is direct but some young are planktonic


How do cephalopods seize prey?

Prehensile tentacles hold it with their 8 arms that contain suckers.

Octopods lack tentacles

Nautilus have about 90 non-suckered tentacles. Only some of which are capable of grasping (prehensile)


How do cephalopods eat?

Prey is dispatched with horny, parrot-like beak and poison glands.

Flesh of prey is torn apart by radula.

Digestive system is adapted for rapid digestion


What are some cephalopod adaptations that are similar to that of vertebrates?

Secondary folded gills

Absence of gill cilia

Blood-vascular system with arteries, veins and capillaries

Accessory brachial hearts


Highly developed eyes

Complex nervous system and behaviour


Ink gland