Mollusca (Snails, Clams and Squid)

Phylum Mollusca (Snails, Clams, and Squids)

• Includes snails, clams, squids, etc.
• Mostly marine, some freshwater and terrestrial
• Have calcareous shells for protection
• Many have lost these shells secondarily

Characteristics

• Mantle: secretes shell
• Ventral foot: large muscular foot used for movement
• Organs: jumbled visceral mass above foot
• Radula: mouthparts used for scraping
• Gills: used for gas exchange or feeding Example: bivalves (clams) use gills to filter water for food
• Open circulatory system
• Reduced Coelom
• Lack segmentation
• Trochophores (free swimming larvae)

Class Gastropoda

• Snails
• Movement
• Glide along with muscular contractions of the large foot
• Most have single, coiled, tubular shell
• Except slugs which have no shell
• Those with shells undergo torsion
• Anus is twisted to the front (above the head)
• Mostly marine but many freshwater and terrestrial
• Trochophore larvae
• Gas exchange via lungs
• Grazers
• Nudibranchs: marine gastropods with no shell
• Lots of color
• Defense using nematocysts stolen from cnidarian prey

Class Bivalvia

• Clams, oysters, scallops
• Humans use them for
• Food
• Jewelry
• Pearls
• Some are invasive pests
• Mostly marine
• Two hinged shells ("bivalve")
• Feeding
• Filter water through gills
• Lack radula
• Lack head
• Some are sessile others are sedentary Example: Scallops can move by "clapping" their shells

Class Cephalopoda

• Squids, cuttlefish, octopuses
• Large bodies
• Low diversity
• Have long tentacles around mouth
• Tentacles have suckers and hooks
• All are predatory
• Radula is modified into a biting beak
• Some have poison
• Lack external shell
• Except nautiluses and extinct ammonites
• Some have Internal shell remnants Example: Cuttlefish bone
• Siphon
• Made from a tube of mantle
• Used for jet propulsion
• Closed circulatory system
• Only mulloscs with it
• Support active lifestyle
• Reproduction
• Attach egg cases to substrates
• Maternal Care
• Lack trochophore larvae
• Direct-developing juveniles
• Excellent vision
• Complex behavior
• Communicate with color and posture
• Very intelligent!