 Contains over 100,000 species, making it the 2 nd largest animal phylum  Includes: snails, chitons, clams, slugs and squids

 Snails, clams and squid look nothing alike, but they have a few shared features  These common features are:  Foot  Mantle  Radula

 A muscular organ, shaped and used differently by different species  Used for digging, grasping, or creeping  Examples:  Pelecypoda (Bivalvia) – “hatchet – shaped foot”  Clams & Oysters - use the foot for digging  Gastropoda – “Stomach foot”  Snails – use the foot to crawl; stomach is in the foot  Cephalopoda – “Head foot”  Octopi and Squids – foot is modified into tentacles that attach to the head; used for moving, grasping, holding, sinking ships…

 The mantle produces the shell and creates the color patterns in most mollusks  The shell is an exoskeleton, even if its covered, and grows with the mollusk  Examples:  A chiton’s mantle produces jointed plates that allow it to curl up into a ball and to move flexibly  Bivalves produce two shells hinged at the top  Snails (Gastropoda) produce a single, spiral-shaped shell  The mantle is inside the shell, so it can’t be seen  In other types of gastropods the mantle covers the outside of the shell, making it look shiny and new in appearance  In Cephalopods and some gastropods (like sea hares), the shell is very small and the mantle completely covers it  Nudibranchs (sea slugs) are shell-less gastropods

 See Fig 6-2 & 6-3  Radula – specially adapted rasp-like tongue  Proboscis – an extension of the mouth  Examples:  Herbivorous snails have a mouth with radula containing many rows, each with 5-7 complex teeth  Use the radula like a file  Carnivorous snails (ex: cone shells) have hollow, barbed, radular teeth (like harpoons) that they thrust into their prey and inject venom through them  These barbs shoot out through the proboscis  Some cone shell venom is strong enough to kill humans

 Foot  Clams – their hatchet-shaped foot is used for moving and burrowing in mud or sand  Oyster & mussels – smaller foot used for attaching themselves to a hard substrate  Scallops – don’t use foot for moving; they clap their shells together to move by jet propulsion  Siphons  Two tubes that allow water to enter and exit the bivalve  Water coming in is oxygen and nutrient rich  The water flows across the gills, O2 and CO2 are exchanged and food particles are trapped by mucus  Cilia  Tiny, hair-like structures that move trapped food towards the mouth  Located on the gills  Palps  Lip-like structures help sort the food and direct it to the mouth * See Table 6-1 for a glossary for more bivalve anatomy

 Pearls  In some bivalves (like oysters) irritating particles (indigestible food particles and sand) get lodged between the shell and mantle  The bivalve will then begin secreting a pearly substance called nacre  These layers of nacre eventually form pearls  Food  Many bivalves are valued as a food source  They can be very sensitive to the environment; they themselves are not sensitive to the toxicity; but they absorb it into their tissues which is then transferred to the consumer