Lecture 4 RH Flashcards
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?
Monoplacophora
Polyplacophora - chitons gastropods
Bivalves
Cephalopods
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?
digestive
circulatory
nervous
muscular
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?
ventrally
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?
metanephridia
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?
limpets
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