Lecture 13 Flashcards
Gastropods
- coelomate bilaterians
- gills in mantle cavity
- ventral creeping foot
- have terminal mouth with chitinous radula
- coelom scanty
- circulation open
- cephalized, nerve cords with nerve ring or brain
- mantle secretes calcareous shell
- extremely abundant
Radula
- filter feeding apparatus
- rasping organ that reduces any prey to small fragments passed to the pharynx
Foot
- attaches the animal firmly to the substrate (during locomotion or at rest)
- moves the animal by waves of muscular contraction
Foot: During locomotive
weak adhesive force needed
- gel secreted by foot, provides no strength in shear but enables to adhere to the substrate by suction
Foot: At rest
- requires strong adhesive force
- secreted gel acts as a glue bonding the foot firmly to the substrate
- Gel consists of polymers
Mantle and shell
- tough organic outer layer
- shell is secreted by mantle
- calcium is very abundant (make shell)
3 layers of shell
Periostracum (Outermost)
Prismatic (Middle)
Nacreous (innermost)
Periostracum
tough, organic, made of chitin
- where growth occurs
Prismatic
calcium carbonate as calcite
nacreous
calcium carbonate as aragonite
Shell - Columella
elongated cone wound into a spiral around a central axis
an elongated uncoiled shell would be impossible to carry because of its high centre of gravity
Nervous System - Mollusca
nervous system consists of ganglia and nerve cords
brain consists of 3 pairs of ganglia located close to the oesophagus
1. Cerebral ganglia
2. Pedal ganglia
3. Pleural ganglia
Mollusca - Cerebral ganglia
located above the oesophagus, supply nerves to eyes, tentacles and other (in head)
Mollusca - Pedal ganglia
Supply nerves to the foot muscles
Mollusca - Pleural ganglia
supply nerves to the mantle cavity, run above and below the oesophagus to connect the right and left cerebral and pedal ganglia to each other
Buccal Ganglia
supply nerves to the mouth and radula
Resting and action potential
worked out in squid giant axon
Nerves
- very large axons
- sponges are the only that do not have nervous system
- ganglia: local concentration of nerve tissues
- Nerve net: sea anemone
- Ganglia and nerves: earthworm and squids
- CNS and PNS: humans
- Neurons vary in size and shape depending on function
A generalized neuron
Dendrites: receives info from other neurons via synapses
Cell body (soma): contains nucleus and most cell organelles
Axon Hillock: integrates info collected by dendrites and initiates action potentials
Axon: telephone line of nerve cell that synapses at terminal to target cell (other neuron, gland or muscle cell)
Neurons:
generate and transmit electrical signals
Glial cells (6)
- electrical insulation of nerve cells
- blood-brain barrier
- uptake of transmitter
- provide nutrients, O2
- maintain extracellular environment
- fight infections of the nervous system
# of glial cells = # of nerve cells
Neurons work electrochemically
“Electro”: difference in charge across the cell (neuron) membrane
“Chemical”: concentration gradients of ions across cell membrane
Lipid Bilayer prevents free movement on ions
Ion channel and ion transporter prevents ion flux across membrane
Membranes and membrane potentials
Cells are surrounded by Lipid membranes = ions normally can’t pass through them - allows cells to change ion composition of their interior cytoplasm relative to their surrounding
ion pumps are used to bring in/pump out ions - they are specialized to transport specific ions across their membrane
their active process requires E from ATP
High concentration of Na+ outside and K+ inside
Voltage-gated ion channels
Closed-Open - inactivated channels
channels are ion-selective
To open them: reduction in membrane potential caused by a stimulus such as sensory input/transmission of an impulse from a neuron
Resting Potential
Resting membrane potential is caused by differences in the concentrations of ions inside/outside cell
difference of charge inside/outside cell is governed by ionic species (mostly potassium)
Activation: opening of potassium channels
- Excitable cells can use the resting potential through voltage ion channels
- Voltage-gated sodium channels are closed at resting potential. They open for ion flow whne the mmrbane potential shifts above a certain threshold
1. Resting: Na+ and K+ channels closed
2. Depolarization: Na+ channel open
3. Hyperpolarization: Na+ channels close, K+ channels open
Formation of an action potential:
- Stimulus from a sensory cell causes the target cell to depolarize toward the threshold potential
- If the threshold of excitation is reached, all Na+ channels open and the membrane depolarizes
- AT the peak action potential, K+ channels open and K+ begins to leave the cell. Na+ channels close
- Membrane becomes hyperpolarized K+ ions leave cell. Hyperpolarized membrane is in a refractory period (cannot fire)
- K+ channels close and Na+/K+ transporter restores the resting potential
Transmission of signal along the axon
- waves of depolarization is spreading down the axon, which makes stimulus
- transmission of signal along axon: the action potential must travel along the axon and reach the axon terminals, where it can initiate neurotransmitter release
Speed of conduction varies with the tissue
standing wave goes down the axon, to the synapse which transmits the info
Myelin
insulator to prevent current ion from leaving axon, increasing speed of action potential conduction
Synaptic transmission between neurons
synaptic cleft = fluid filled space
neurotransmission at a chemical synapse begins with the arrival of an action potential at the presynaptic axon terminal
an action potential will depolarize the membrane when it reaches the axon. The voltage-gated Na channels are opened, Na ions enter the cell, furthuer depoarlized the presynaptic membrane
- This depolarization causes voltage-gated Ca channels to open. Ca ions enter the cell, initiating a signaling cascade
Eyes - Mollusca
- every grade of complexity, from simple photoreceptor to image forming eyes, is found among molluscs
Larvae
- gastrula develops into a ciliated-band larva
- species with external fertilization: first larval stage is trochophore, giving rise to the characteristic molluscan veliger larva
- Species with internal fertilization: trochophore stage is passed within an egg capsule, and the animal hatches as a veliger
Trochophores
named for the wheel-like appearance of the main ciliary band. Mouth opens just below the prototroph, the anus is posterior and terminal
Gastropods and bivalves
have veliger larvae. It has the basis topology of a trochophore but with an expanded ventral foot region with an operculum (shell secreted to enclose the visceral mass)
Torsion
- during the development of veliger, the visceral mass rotates 180 degrees
- mantle cavity, gills, anus, gonopore, and nephridiopore are now anterior, above the head
- creates difficulties in the disposal of waste over the head
- Advantage: permits a compact body form protected by the shell once the mantle cavity has moved forward
- twisting of gut and crossing of nerve cord is caused my torsion
Metamorphosis
- veliger lives from a few hours to a few weeks in the plankton.
- organs are retained as the larval heart regresses
- larval shell is retained as the protoconch at the tip of the shell spire
Conus
modification of the radula for predation
Opisthobranch Molluscs: Nudibranchs
- Radula is present
- Hermaphroditic - compete male and female systems, internal fertilization
- externally bilaterally symmetrical
- no shells
- instead of being protected, they are poisonous
- Don’t do torsion
- Respiratory structures on their back
- Some nudibranchs lack dorsal projections and resemble polyclad flatworms (very coloured)
- Convergent evolution of pelagic gastropods by modification of the foot: Pterotrachea, Corolla
Pulmonate Gastropods: breathing
- familiar snails and slugs - extremely diverse and abundant in terrestrial and freshwater habitat
- conversion of the mantle cavity
- spirally coiled shell or shell reduced or absent in some terrestrial groups
Bivalvia
- headless mollusks - no jaw or radula
- buried forms communicate with surface through siphons
- mantle cavity contains a pair of large ciliated gills
Scaphopoda
- curved tubular shell, open at both ends
- retractile foot - modified for filamentous outgrowth
- lies buried head down in sand or mud
- ciliated tentacles which probe for food
- lost their heads
Cephalopoda
- squids, octopus
- have nervous and sensory system
- Foot modified into grasping organs
- advanced in image-forming eyes
Nautilus
- restricted to indo-parcific tropics at moderate depth o fringing reefs
- predators on crustaceans - also scanvengers
- locomotion by jet propulsion using funnel
Giant Squid
- part of cephalopod
- Enormous animal
- deepwater predators - predators are sperm whales
Polyplacophora
- have calcareous shell
- have a radula
- present in all seas
- Chitons
Monoplacophora
- became extinct a long time ago and were re-discovered in 1952
- Broad ventral foot
Aplacophora
- body covered with calcareous spicules
