Mollusca Flashcards
To Study the Last Slide pack
Mollusca
- Bilateral coelomates
- Open circulatory system (except cephalopods)
- Body covered by epidermal sheet, mantle
that secretes spicules, sclerites or a shell - Well defined muscular foot
- Mouth with cuticular teeth (radula)
- Complete gut with regional specialization
- Generally spiral cleavage with trochophore
larvae
Part of the Lophotrochozoa -
small shelly fossils- with annelids, nermertines, phoronids, bryozoans and brachiopods
8 Mollusca Classes:
Caudofoveata & Solenogastres, Polyplacophora, Monoplacophora, Scaphopoda, Cephalopoda, Gastropoda, and Bivalvia
Caudofoveata & Solenogastres:
Essentially shell-less molluscan worms
Polyplacophora:
Chitons
Monoplacophora:
Living limpet-like fossils
Scaphopoda:
Tusk shells
Cephalopoda:
Nautiloids, ammonoids, squids & cuttlefish, octopods, and kin
Gastropoda:
Snails and slugs
Bivalvia:
Clams, broadly defined (including cockles, mussels, scallops, and oh so many more)
The Shell -
- Most molluscs have shell(s) made of calcium carbonate in a protein matrix
- Aplacophorans have aragonite sclerites - Secreted by shell glands in the mantle
Shell Morphology and Terms -
Slide 7 - 8*
Gastropod shells -
- Snails twist into their shells along the central axis, columella
- Shell is typically leaning to the left and coils to the right (dextral)
- The ctenidium, ospharadium, nephridia and heart tend to be reduced
Gastropod Torsion -
- Counterclockwise twisting of the visceral mass up to 180° with respect to head and foot during development
- Nervous and digestive systems are twisted, figure 8 config.
- Mantle cavity moves to over the head
Mantle Cavity -
- Lies between mantle and viscera
- Houses the gills, or ctenidia
- May function only in respiration or also sorts food - Exit of excretory, digestive and reproductive systems
Osphradium-
a chemoreceptor or tactile receptor located adjacent to the ctenidium
Gills work through countercurrent exchange –
blood and water flow in opposing directions to enhance gas exchange
Circulatory System -
- Cephalopods have a closed system while the rest are open system
- Blood moves through hemocoel in open system that is pumped by a heart
- Except scaphopods which lack a heart, rely on contractions of the foot
Hemoglobin -
With iron (some bivalves and pulmonates)
Hemocyanin -
With copper(pulmonates, prosobranchs and cephalopods)
Foot -
- Highly variable among the molluscs
- Most form flat, ventral, creeping ciliated sole with mucus glands
Locomotion -
- Ciliary gliding
- Pedal waves
- Swimming
- Sucker crawling (cephalopods)
Ciliary gliding -
Requires a mucus trail over which the animal glides
Pedal waves -
- Muscular contractions of the foot raise and lower sole to cause directional or retrograde waves
- Directional – longitudinal and dorsoventral muscles
- Retrograde- transverse muscles contract to extend anterior portion
Swimming -
Evolved several times, undulations of the body or foot, jet propulsion in cephalopods
The Radula -
- Feeding structure in many molluscs
- Ribbons of chitinous teeth produced from radular sac is supported by the odontophore – cartilage-like structure
Suspension Feeding -
- Modification of ctenidia (V shaped) to trap particulates through the respiratory current
- Lined with compound cirri, or feeding cilia
- Radula is often reduced, used to pull in mucus, or absent in bivalves
- Labial palps used for particle sorting and rejected particles are called pseudofeces
Digestive system -
- Complete, through gut
- Mouth leads to esophagus, then stomach with digestive glands or caeca facilitates extracellular digestion
- Herbivores have a gizzard with teeth to grind up plant matter
Crystalline style –
Rod-like matrix lined with digestive enzymes found in autobranch bivalves and some gastropods
Cerata –
Extensions of digestive gland in nudibranchs, also used in gas exchange
Excretory System -
- Has one or more pairs of kidneys (nephridia)
- Coelomic fluid enters kidney through nephrostome, then into nephridium where selective reabsorption occurs
- Requires pressure filtration, secretion and reabsorption
Nervous System -
- Ganglia range from not present in chitons to well developed in cephalopods (forms true brain)
- Major nerve cords run to the mantle (pleural), foot (pedal), gills (parietal), viscera (visceral) and radula (buccal)
Sensory Organs -
Tentacles, photoreceptors, statocysts, osphradia and eyes
Cephalopod Color and Ink -
- Color and texture of skin used in camouflage and communication
- Pigment cells (chromatophores) expand and contract by muscles at base of the cell to display or hide color
Iridocytes -
prismatic cells that reflect or refract light
Reproduction -
- Most are gonochoristic with indirect development
- Typically paired gonads with exception of gastropods and cephalopods that have one
- Hermaphroditism present in gastropods, exchange spermatophores during copulation
Development -
- Most undergo spiral cleavage
- Indirect development typically has free-swimming trochophore larva, similar to annelids
- Apical tufts and ciliated prototroch
- In gastropods and autobranch bivalves, a second larval stage, the veliger is present
