Mollusk characteristics
• Soft-bodied animals with an internal or external shell
• Trochophore: free-swimming larvae stage• Body plan
– Foot: crawling, burrowing, tentacles– Mantle: thin tissue layer that covers body– Shell: made from glands that secrete calcium
carbonate, may be reduced or lost in some– Visceral mass: internal organs
Section 27-4
Shell
Mantle cavity
Foot
Gills
Digestive tract
Snail
Earlymollusk
Clam
Squid
The Mollusk Body Plan
Figure 27–21
Feeding
• Herbivores, carnivores, filter feeders, detritivores, parasites
• Siphon: tubelike structure where water enters and leaves the body, washing water over gills and trapping plankton
Radula
• flexible, tongue-shaped structure with tiny teeth for drilling through shells or scraping algae off rocks
Hypselodoris bilineata
Respiration
• Gills within mantle cavity
• Large surface area with blood vessels
Orange peel nudibranch (about 25 cm length) with protruding white gills on the ventral side
Excretion and Reproduction
• Nephridia: tube shaped structures remove ammonia from blood
• Release waste outside of body
• Sexual reproduction
• External fertilization
Response
• Clams and other bivalves live sedentary lives and have simple nervous systems.
• Cephalopods have highly developed nervous systems.
octopus eye
Movement
• Snails: secrete mucus and crawl slowly
• Cephalopods: fast moving, drawing water into mantle cavity and forcing water out of siphon (like jet propulsion)
excessive amount of snail mucus secreted
Section 27-4
Mouth
Shell
Stomach CoelomHeart
Nephridium
Adductor muscle
Anus
Excurrentsiphon
Incurrentsiphon
Gills
Mantle cavity
Foot
Intestine
Mantle cavity
Adductormuscle
The Anatomy of a Clam
Figure 27–23
Groups of mollusks
• Cephalopods: soft-bodied mollusks where the head is attached to a foot, divided into arms and tentacles