BIO 5

• The phylum Mollusca includes 150,000 known species of diverse forms, including snails and slugs, oysters and clams, and octopuses and squids
• Most mollusks are marine though some inhabit fresh water, and some snails and slugs live on land
• Mollusks are soft-bodied animals, but most are protected by a hard shell of calcium carbonate
• Slugs, squids, and octopuses have reduced or lost their shells completely during their evolution
• Mollusks


  • Have a muscular foot (typically for locomotion)
  • Have a visceral mass with most of the internal organs
  • Have a mantle
• Mantle


  Drapes over the visceral mass and creates a water-filled chamber, the mantle cavity, with the gills, anus, and excretory pores
• Radula


  A straplike rasping organ used by many mollusks to scrape up food
• Mollusks are coelomates have a high level of chepalisation and show complex behavior
• Typical features of Molluscs

  • Head and foot complex with nerves and locomotory organs
  • Visceral hump contains organs for digestion, reproduction, excretion
  • Mantle hanging form visceral mass and secreting shell
  • Space between mantle shell and visceral mass - mantle cavity. Contains gills (ctenidia) and canals from alimentary canal, excretory and genital system

  
• Gastropod


  Shell in 1 piece eg limpets, snails
• Bivalvia


  Shell forms 2 valves, eg mussels clams etc
• Cephalopoda


  eg squid, octopus – have no external shell (squids have a rudimentry internal shell)

  
• Most mollusks have separate sexes, with gonads located in the visceral mass
• Many snails are outcrossing hermaphrodites
• The life cycle of many marine mollusks includes a ciliated larvae, the trophophore
• The trophophore larva is also found in marine annelids (segmented worms)
• The basic molluscan body plan


  • Has evolved in various ways in the eight classes of the phylum

  
• The four most prominent classes

  • Polyplacophora (chitons)
  • Gastropoda (snails and slugs)
  • Bivalvia (clams, oysters, and other bivalves)
  • Cephalopoda (squids, octopuses, and nautiluses)

  
• Chitons


  Marine animals with oval shapes and shells divided into eight dorsal plates
• Chitons


  • Use their muscular foot to grip the rocky substrate tightly and to creep slowly over the rock surface
  • Are grazers that use their radulas to scrape and ingest algae
• Most of the more than 40,000 species in the Gastropoda are marine, but there are also many freshwater species
• Garden snails and slugs have adapted to land
• During embryonic development, gastropods undergo 40,000 in which the visceral mass is rotated up to 180 degrees, such that the anus and mantle cavity are above the head in adults

  
• Gastropod shells


  • Typically conical, but those of abalones and limpets are somewhat flattened
• Other gastropod species have lost their shells entirely and may have chemical defenses against predators
• Gastropods


  • Have distinct heads with eyes at the tips of tentacles
  • Move by a rippling motion of their foot
  • Most use their radula to graze on algae or plant material
  • Some species are predators and have a modified radula to bore holes in the shells of other organisms or to tear apart tough animal tissues

  
• In the tropical marine cone snails, teeth on the radula form separate poison darts, which penetrate and stun their prey, including fishes
• Gastropods are among the few invertebrate groups to have successfully populated the land
• Terrestrial snail "lung"


  The lining of the mantle cavity functions as a "lung"
• Gastropod foot


  • Has a flat creeping sole which has become adapted for locomotion over a variety of substrata
  • Is ciliated and is provided with numerous gland cells which produce mucus (pedal gland)
  • Small snails as well as those that live on sand and mud move mainly by ciliary propulsion
  • Has a complex set of muscles that work against a haemocoele hydrostatic skeleton – blood spaces within the tissue

  
• Gastropod movement


  1. Achieved by waves of muscular contractions passing along the foot

  2. Foot remains in contact with the substratum by mucus

  3. Direct - waves pass the same direction as movement of the animal

  4. Retrograde - waves pass opposite direction to the movement

  5. Ditactic - waves of muscular contraction occur on alternating sides of the foot

  
• Bivalvia


  Includes clams, oysters, mussels, and scallops
• Bivalve shells


  • Divided into two halves, hinged at the mid-dorsal line, with powerful adductor muscles to close the shell tightly
• Bivalve mantle cavity


  • Contains gills that are used for feeding and gas exchange
• Bivalve feeding


  Most are suspension feeders, trapping fine particles in mucus that coats the gills, with cilia conveying the particles to the mouth
• Bivalve movement


  • Most live rather sedentary lives, with sessile mussels secrete strong threads that tether them to rocks, docks, boats, and the shells of other animals
  • Clams can pull themselves into the sand or mud, using the muscular foot as an anchor
  • Scallops can swim in short bursts to avoid predators by flapping their shells and jetting water out their mantle cavity
• Bivalve foot


  • Specialized for burrowing, achieved by the interaction of the foot muscles (hydrostatic skeleton) and the shell (external skeleton)
• Cephalopods


  • Use rapid movements to dart toward their prey which they capture with several long tentacles
  • Squids and octopuses use beaklike jaws to bite their prey and then inject poison to immobilize the victim
• A mantle covers the visceral mass, but the shell is reduced and internal in squids, missing in many octopuses, and exists externally only in nautiluses

  
• Squid movement


  Contracts its mantle cavity and fires a stream of water through the excurrent siphon, pointing the siphon in different directions to rapidly move in different directions